<p class="course-title-kicker">CS103 · Lecture 18</p> # Destructors, Copying, and RAII <p class="course-title-tagline">An object keeps its invariant from construction to destruction</p> <p class="course-title-instructor">Dr. Debasish Pattanayak · <a href="https://drdebmath.github.io">drdebmath.github.io</a></p>

## Where Lecture 17 ended · The constructor establishes the invariant A **constructor** makes an object valid (Lecture 17). Every `new` needs one owner and one release (Lecture 14). But objects are also copied, assigned, and destroyed. **Today's question.** Who keeps the invariant then, and who releases what the object owns?

## Destructors · Last constructed, first destroyed ~~~cpp class Tag { public: Tag(std::string n) : name(n) { std::cout << "construct " << name << '\n'; } ~Tag() { std::cout << "destroy " << name << '\n'; } private: std::string name; }; int main() { Tag a("a"); { Tag b("b"); Tag c("c"); std::cout << "end of block\n"; } std::cout << "end of main\n"; } // construct a, construct b, construct c, end of block, // destroy c, destroy b, end of main, destroy a ~~~

## Destructors step by step <figure class="course-stepper" data-course-stepper> <svg class="course-array-address-figure course-l18life-figure" viewBox="0 0 900 380" role="img" aria-label="A step-by-step trace of a class Tag whose constructor prints construct and whose destructor prints destroy, in 9 frames. Frame 1: in main, object a is constructed and the output shows construct a. Frame 2: inside the inner block, b is constructed and the output adds construct b. Frame 3: c is constructed and the output adds construct c. Frame 4: the program prints end of block and no destructor has run. Frame 5: the closing brace of the inner block ends the block and c, the last constructed, is destroyed first, printing destroy c. Frame 6: b is destroyed next, printing destroy b, in the reverse order of construction. Frame 7: the inner block is gone, a is still alive, and the output adds end of main. Frame 8: the closing brace of main destroys a, printing destroy a. Frame 9: summary: objects were destroyed in the order c, b, a, last constructed first destroyed, and nobody wrote a call to the destructor." xmlns="http://www.w3.org/2000/svg"> <style> .course-l18life-figure text{font-family:ui-monospace,Menlo,Consolas,monospace;fill:currentColor} .course-l18life-figure .cell{fill:none;stroke:currentColor;stroke-width:2} .course-l18life-figure .cell.obj{fill:rgba(142,232,216,.18)} .course-l18life-figure .cell.heap{fill:rgba(255,214,102,.22)} .course-l18life-figure .cell.dead{fill:rgba(255,120,120,.22);stroke-dasharray:6 5} .course-l18life-figure .hl{fill:rgba(110,170,255,.35);stroke:none} .course-l18life-figure .arrow{fill:none;stroke:currentColor;stroke-width:2.5} .course-l18life-figure .head{fill:currentColor} </style> <g data-step="0"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="hl" x="35" y="95" width="110" height="60"/><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">Tag a("a");</text><text x="450" y="362" font-size="16" text-anchor="middle">The constructor runs as a is created and prints construct a</text></g> <g data-step="1"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell" x="170" y="96" width="410" height="186" style="stroke-dasharray:3 6"/><text x="186" y="122" font-size="18" font-weight="700">inner block</text><rect class="hl" x="185" y="135" width="110" height="60"/><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell obj" x="190" y="140" width="100" height="50"/><text x="240" y="174" font-size="22" text-anchor="middle">b</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">Tag b("b");</text><text x="450" y="362" font-size="16" text-anchor="middle">Entering the inner block: b is constructed second</text></g> <g data-step="2"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell" x="170" y="96" width="410" height="186" style="stroke-dasharray:3 6"/><text x="186" y="122" font-size="18" font-weight="700">inner block</text><rect class="hl" x="315" y="135" width="110" height="60"/><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell obj" x="190" y="140" width="100" height="50"/><text x="240" y="174" font-size="22" text-anchor="middle">b</text><rect class="cell obj" x="320" y="140" width="100" height="50"/><text x="370" y="174" font-size="22" text-anchor="middle">c</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">Tag c("c");</text><text x="450" y="362" font-size="16" text-anchor="middle">c is constructed third; all three objects are alive</text></g> <g data-step="3"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell" x="170" y="96" width="410" height="186" style="stroke-dasharray:3 6"/><text x="186" y="122" font-size="18" font-weight="700">inner block</text><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell obj" x="190" y="140" width="100" height="50"/><text x="240" y="174" font-size="22" text-anchor="middle">b</text><rect class="cell obj" x="320" y="140" width="100" height="50"/><text x="370" y="174" font-size="22" text-anchor="middle">c</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << "end of block\n";</text><text x="450" y="362" font-size="16" text-anchor="middle">Ordinary code runs; no destructor has been called yet</text></g> <g data-step="4"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell" x="170" y="96" width="410" height="186" style="stroke-dasharray:3 6"/><text x="186" y="122" font-size="18" font-weight="700">inner block</text><rect class="hl" x="315" y="135" width="110" height="60"/><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell obj" x="190" y="140" width="100" height="50"/><text x="240" y="174" font-size="22" text-anchor="middle">b</text><rect class="cell dead" x="320" y="140" width="100" height="50"/><text x="370" y="174" font-size="22" text-anchor="middle">c</text><text x="370" y="212" font-size="15" text-anchor="middle">destroyed</text><text x="186" y="250" font-size="16">c is destroyed first</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="636" y="192" font-size="16">destroy c</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // closing brace</text><text x="450" y="362" font-size="16" text-anchor="middle">The block ends: c, the last constructed, is destroyed first</text></g> <g data-step="5"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell" x="170" y="96" width="410" height="186" style="stroke-dasharray:3 6"/><text x="186" y="122" font-size="18" font-weight="700">inner block</text><rect class="hl" x="185" y="135" width="110" height="60"/><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell dead" x="190" y="140" width="100" height="50"/><text x="240" y="174" font-size="22" text-anchor="middle">b</text><text x="240" y="212" font-size="15" text-anchor="middle">destroyed</text><rect class="cell dead" x="320" y="140" width="100" height="50"/><text x="370" y="174" font-size="22" text-anchor="middle">c</text><text x="370" y="212" font-size="15" text-anchor="middle">destroyed</text><text x="186" y="250" font-size="16">then b: reverse of construction</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="636" y="192" font-size="16">destroy c</text><text x="636" y="214" font-size="16">destroy b</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // closing brace</text><text x="450" y="362" font-size="16" text-anchor="middle">Then b is destroyed; a is untouched because main has not ended</text></g> <g data-step="6"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell obj" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="636" y="192" font-size="16">destroy c</text><text x="636" y="214" font-size="16">destroy b</text><text x="636" y="236" font-size="16">end of main</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << "end of main\n";</text><text x="450" y="362" font-size="16" text-anchor="middle">b and c are gone; a is still alive in main</text></g> <g data-step="7"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="hl" x="35" y="95" width="110" height="60"/><rect class="cell dead" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><text x="90" y="172" font-size="15" text-anchor="middle">destroyed</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="636" y="192" font-size="16">destroy c</text><text x="636" y="214" font-size="16">destroy b</text><text x="636" y="236" font-size="16">end of main</text><text x="636" y="258" font-size="16">destroy a</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // end of main</text><text x="450" y="362" font-size="16" text-anchor="middle">The closing brace of main ends a's lifetime: destroy a</text></g> <g data-step="8"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 construct</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 inner block ends</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 main ends</tspan></text><rect class="cell" x="20" y="52" width="580" height="240" style="stroke-dasharray:3 6"/><text x="36" y="80" font-size="18" font-weight="700">main</text><rect class="cell dead" x="40" y="100" width="100" height="50"/><text x="90" y="134" font-size="22" text-anchor="middle">a</text><text x="90" y="172" font-size="15" text-anchor="middle">destroyed</text><text x="36" y="230" font-size="16">destroyed: c, then b, then a</text><rect class="cell" x="620" y="52" width="260" height="240"/><text x="636" y="80" font-size="18" font-weight="700">output</text><text x="636" y="104" font-size="16">construct a</text><text x="636" y="126" font-size="16">construct b</text><text x="636" y="148" font-size="16">construct c</text><text x="636" y="170" font-size="16">end of block</text><text x="636" y="192" font-size="16">destroy c</text><text x="636" y="214" font-size="16">destroy b</text><text x="636" y="236" font-size="16">end of main</text><text x="636" y="258" font-size="16">destroy a</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">no call to ~Tag() appears in main</text><text x="450" y="362" font-size="16" text-anchor="middle">Last constructed, first destroyed; nobody called the destructor</text></g> </svg> </figure>

## The Big Three: the general form | Part | Meaning | | --- | --- | | `~Name() { ... }` | The **destructor**: no return type, no parameters, one per class. It runs when the object's lifetime ends. | | `Name(const Name& other)` | The **copy constructor**: creates a new object from an existing one. | | `Name& operator=(const Name& other)` | **Copy assignment**, an overloaded operator: replaces the value of an object that already exists. | | `this` | A pointer to the object the member function was called on; `this == &other` detects `a = a`. | | `return *this;` | Copy assignment returns the object itself, so `a = b = c` works. | | Not written? | The compiler generates all three; they copy or destroy each data member in turn. |

## Poll 1: What does main print? <div class="course-poll-layout"> <div> ~~~cpp class T { public: T(char c) : s(c) { std::cout << '+' << s << ' '; } ~T() { std::cout << '-' << s << ' '; } private: char s; }; int main() { T x('x'); { T y('y'); } T z('z'); } ~~~ **A)** `+x +y -y +z -z -x` · **B)** `+x +y +z -z -y -x`<br>**C)** `+x +y -y +z -x -z` · **D)** `+x +y +z -x -y -z` </div> <figure class="course-slido-qr"><img src="assets/slido-lecture18.svg" alt="QR code that opens the Lecture 18 Slido event." width="360" height="360"><figcaption>📊 sli.do · #4174771</figcaption></figure> </div>

## Shallow copy · Two owners, one array ~~~cpp class IntArray { public: IntArray(int n) : data(new int[n]{}) {} ~IntArray() { std::cout << "delete[] " << data << std::endl; delete[] data; } void set(int i, int v) { data[i] = v; } int get(int i) const { return data[i]; } private: int* data; // invariant: this object alone owns the array }; int main() { IntArray a(3); IntArray b = a; // shallow copy: b.data == a.data b.set(0, 99); std::cout << a.get(0) << '\n'; // a changed too } // prints 99, then "delete[] 0x1028b1b20" twice; the program aborts ~~~

## Shallow copy step by step <figure class="course-stepper" data-course-stepper> <svg class="course-array-address-figure course-l18shallow-figure" viewBox="0 0 900 380" role="img" aria-label="A step-by-step trace of class IntArray, whose data member is a pointer to an array that the object alone owns, in 7 frames. Frame 1: IntArray a(3) runs the constructor, which allocates an array of three zeros on the free store, and a.data points to it. Frame 2: IntArray b = a uses the compiler-generated copy, which copies the pointer value only, so b.data and a.data both point to the same array, labeled two owners. Frame 3: b.set(0, 99) writes through the pointer and element 0 of the shared array becomes 99. Frame 4: a.get(0) prints 99, so a changed although only b was written; the output is 99. Frame 5: at the closing brace the destructor of b runs first, prints the address and runs delete[], so the array becomes a dashed red freed block and a.data dangles. Frame 6: the destructor of a prints the same address and deletes the same array again, a double deletion, which is undefined behavior; the program aborts and AddressSanitizer reports attempting double-free. Frame 7: the summary shows both objects pointing to one array: the invariant that an object alone owns its array was broken by the shallow copy the compiler generated, not by any member function." xmlns="http://www.w3.org/2000/svg"> <style> .course-l18shallow-figure text{font-family:ui-monospace,Menlo,Consolas,monospace;fill:currentColor} .course-l18shallow-figure .cell{fill:none;stroke:currentColor;stroke-width:2} .course-l18shallow-figure .cell.obj{fill:rgba(142,232,216,.18)} .course-l18shallow-figure .cell.heap{fill:rgba(255,214,102,.22)} .course-l18shallow-figure .cell.dead{fill:rgba(255,120,120,.22);stroke-dasharray:6 5} .course-l18shallow-figure .hl{fill:rgba(110,170,255,.35);stroke:none} .course-l18shallow-figure .arrow{fill:none;stroke:currentColor;stroke-width:2.5} .course-l18shallow-figure .head{fill:currentColor} </style> <g data-step="0"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><path class="arrow" d="M220 114 H400 V134 H508"/><polygon class="head" points="520,134 506,127 506,141"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="cell heap" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">0</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell heap" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell heap" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray a(3);</text><text x="450" y="362" font-size="16" text-anchor="middle">The constructor allocates int[3] on the free store; a alone owns it</text></g> <g data-step="1"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><rect class="hl" x="24" y="162" width="262" height="82"/><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text><path class="arrow" d="M220 114 H400 V134 H508"/><polygon class="head" points="520,134 506,127 506,141"/><path class="arrow" d="M220 204 H400 V154 H508"/><polygon class="head" points="520,154 506,147 506,161"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="cell heap" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">0</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell heap" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell heap" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">two owners</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray b = a;</text><text x="450" y="362" font-size="16" text-anchor="middle">Shallow copy: the pointer is copied, not the array, so b.data points to it too</text></g> <g data-step="2"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text><path class="arrow" d="M220 114 H400 V134 H508"/><polygon class="head" points="520,134 506,127 506,141"/><path class="arrow" d="M220 204 H400 V154 H508"/><polygon class="head" points="520,154 506,147 506,161"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="hl" x="515" y="115" width="90" height="58"/><rect class="cell heap" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">99</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell heap" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell heap" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">two owners</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">b.set(0, 99);</text><text x="450" y="362" font-size="16" text-anchor="middle">b.set writes through its pointer, so element 0 of the one shared array becomes 99</text></g> <g data-step="3"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text><path class="arrow" d="M220 114 H400 V134 H508"/><polygon class="head" points="520,134 506,127 506,141"/><path class="arrow" d="M220 204 H400 V154 H508"/><polygon class="head" points="520,154 506,147 506,161"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="hl" x="515" y="115" width="90" height="58"/><rect class="cell heap" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">99</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell heap" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell heap" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">two owners</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: 99</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << a.get(0) << '\n';</text><text x="450" y="362" font-size="16" text-anchor="middle">a never called set, yet a.get(0) prints 99: a and b see the same array</text></g> <g data-step="4"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><g opacity=".35"><rect class="hl" x="24" y="162" width="262" height="82"/><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text></g><path class="arrow" d="M220 114 H400 V134 H508" style="stroke-dasharray:6 5"/><polygon class="head" points="520,134 506,127 506,141"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="cell dead" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">99</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell dead" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell dead" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="400" y="100" font-size="16" text-anchor="middle" font-weight="700">a.data dangles</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">freed</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: 99, delete[] 0x1028b1b20</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // ~IntArray() runs for b</text><text x="450" y="362" font-size="16" text-anchor="middle">b is destroyed first and frees the array; a.data now dangles</text></g> <g data-step="5"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="hl" x="24" y="72" width="262" height="82"/><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><g opacity=".35"><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text></g><path class="arrow" d="M220 114 H400 V134 H508" style="stroke-dasharray:6 5"/><polygon class="head" points="520,134 506,127 506,141"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="cell dead" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">99</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell dead" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell dead" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">double deletion</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: 99, delete[] 0x1028b1b20, delete[] 0x1028b1b20, then abort</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // ~IntArray() runs for a</text><text x="450" y="362" font-size="16" text-anchor="middle">a deletes the same array again: double deletion, undefined behavior</text></g> <g data-step="6"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 write and read</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 destroy</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">5 lesson</tspan></text><text x="30" y="66" font-size="18" font-weight="700">stack</text><rect class="cell" x="30" y="78" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="100" font-size="18" font-weight="700">a</text><text x="42" y="132" font-size="16">data</text><rect class="cell obj" x="120" y="92" width="100" height="44"/><text x="170" y="121" font-size="16" text-anchor="middle">0x…b20</text><rect class="cell" x="30" y="168" width="250" height="70" style="stroke-dasharray:3 6"/><text x="42" y="190" font-size="18" font-weight="700">b</text><text x="42" y="222" font-size="16">data</text><rect class="cell obj" x="120" y="182" width="100" height="44"/><text x="170" y="211" font-size="16" text-anchor="middle">0x…b20</text><path class="arrow" d="M220 114 H400 V134 H508"/><polygon class="head" points="520,134 506,127 506,141"/><path class="arrow" d="M220 204 H400 V154 H508"/><polygon class="head" points="520,154 506,147 506,161"/><text x="520" y="66" font-size="18" font-weight="700">free store</text><rect class="cell heap" x="520" y="120" width="80" height="48"/><text x="560" y="152" font-size="22" text-anchor="middle">99</text><text x="560" y="190" font-size="15" text-anchor="middle">[0]</text><rect class="cell heap" x="600" y="120" width="80" height="48"/><text x="640" y="152" font-size="22" text-anchor="middle">0</text><text x="640" y="190" font-size="15" text-anchor="middle">[1]</text><rect class="cell heap" x="680" y="120" width="80" height="48"/><text x="720" y="152" font-size="22" text-anchor="middle">0</text><text x="720" y="190" font-size="15" text-anchor="middle">[2]</text><text x="640" y="214" font-size="16" text-anchor="middle" font-weight="700">invariant broken: two owners</text><rect class="cell" x="20" y="252" width="860" height="40" style="stroke-dasharray:3 6"/><text x="36" y="278" font-size="16">printed: 99, delete[] 0x1028b1b20, delete[] 0x1028b1b20, then abort</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray b = a; // the shallow copy broke the invariant</text><text x="450" y="362" font-size="16" text-anchor="middle">No member function broke "this object alone owns"; the compiler-generated copy did</text></g> </svg> </figure>

## A double free in zlib · One block of memory, released twice - zlib 1.1.3, a C compression library in many programs, could free the same memory twice on some invalid input. - CERT/CC advisory CA-2002-07 (12 March 2002): "denial of service, information leakage, or execution of arbitrary code". - zlib 1.1.4 fixed it; every program linked to the old version needed the fix. **Lesson:** give each resource one owner, whose destructor releases it once.

## Copy constructor and copy assignment · A deep copy for each object ~~~cpp IntArray(const IntArray& other) : size(other.size), data(new int[other.size]) { std::copy(other.data, other.data + size, data); // copy constructor } IntArray& operator=(const IntArray& other) { // copy assignment if (this == &other) return *this; // a = a: nothing to do int* fresh = new int[other.size]; std::copy(other.data, other.data + other.size, fresh); delete[] data; data = fresh; size = other.size; // release old array return *this; } // IntArray a(3); IntArray b = a; IntArray c(5); c = a; b.set(0, 99); → 0 99 0 ~~~

## Copy operations step by step <figure class="course-stepper" data-course-stepper> <svg class="course-array-address-figure course-l18deep-figure" viewBox="0 0 900 380" role="img" aria-label="A step-by-step trace of a class IntArray with data members size and data, a pointer to an array on the free store that the object alone owns, in 9 frames. Frame 1: IntArray a(3) gives a a new array of 3 zeros. Frame 2: IntArray b = a runs the copy constructor, which creates the new object b, allocates a separate array and copies the values, so b points to its own array. Frame 3: IntArray c(5) gives c its own array of 5 zeros. Frame 4: c = a runs copy assignment on the existing object c; this == &other is false, so it allocates a fresh array of 3 ints and copies a's values while c.data still points to the old 5-int array. Frame 5: delete[] data releases c's old array, drawn as a dashed freed block. Frame 6: data = fresh and size = 3 make c point to the fresh array. Frame 7: b.set(0, 99) changes only b's array. Frame 8: the program prints 0 99 0. Frame 9: three objects own three arrays, one owner each, and at the closing brace each destructor deletes its own array once." xmlns="http://www.w3.org/2000/svg"> <style> .course-l18deep-figure text{font-family:ui-monospace,Menlo,Consolas,monospace;fill:currentColor} .course-l18deep-figure .cell{fill:none;stroke:currentColor;stroke-width:2} .course-l18deep-figure .cell.obj{fill:rgba(142,232,216,.18)} .course-l18deep-figure .cell.heap{fill:rgba(255,214,102,.22)} .course-l18deep-figure .cell.dead{fill:rgba(255,120,120,.22);stroke-dasharray:6 5} .course-l18deep-figure .hl{fill:rgba(110,170,255,.35);stroke:none} .course-l18deep-figure .arrow{fill:none;stroke:currentColor;stroke-width:2.5} .course-l18deep-figure .head{fill:currentColor} </style> <g data-step="0"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="58" width="68" height="44"/><rect class="hl" x="576" y="58" width="68" height="44"/><rect class="hl" x="636" y="58" width="68" height="44"/><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><text x="20" y="308" font-size="18">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray a(3);</text><text x="450" y="362" font-size="16" text-anchor="middle">a owns a new array of 3 ints on the free store, all zero</text></g> <g data-step="1"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="hl" x="28" y="152" width="108" height="36"/><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="130" width="68" height="44"/><rect class="hl" x="576" y="130" width="68" height="44"/><rect class="hl" x="636" y="130" width="68" height="44"/><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><text x="20" y="308" font-size="18">printed: </text><text x="880" y="308" font-size="16" text-anchor="end">copy constructor runs</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray b = a;</text><text x="450" y="362" font-size="16" text-anchor="middle">b is a new object: the copy constructor allocates its own array and copies a's values</text></g> <g data-step="2"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 5</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="200" width="68" height="44"/><rect class="hl" x="576" y="200" width="68" height="44"/><rect class="hl" x="636" y="200" width="68" height="44"/><rect class="hl" x="696" y="200" width="68" height="44"/><rect class="hl" x="756" y="200" width="68" height="44"/><rect class="cell heap" x="520" y="204" width="60" height="36"/><text x="550" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="204" width="60" height="36"/><text x="610" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="204" width="60" height="36"/><text x="670" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="700" y="204" width="60" height="36"/><text x="730" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="760" y="204" width="60" height="36"/><text x="790" y="229" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V222 H506"/><polygon class="head" points="520,222 506,215 506,229"/><text x="20" y="308" font-size="18">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">IntArray c(5);</text><text x="450" y="362" font-size="16" text-anchor="middle">c owns its own array of 5 ints</text></g> <g data-step="3"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 5</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="204" width="60" height="36"/><text x="550" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="204" width="60" height="36"/><text x="610" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="204" width="60" height="36"/><text x="670" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="700" y="204" width="60" height="36"/><text x="730" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="760" y="204" width="60" height="36"/><text x="790" y="229" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V222 H506"/><polygon class="head" points="520,222 506,215 506,229"/><rect class="hl" x="516" y="246" width="68" height="44"/><rect class="hl" x="576" y="246" width="68" height="44"/><rect class="hl" x="636" y="246" width="68" height="44"/><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><text x="430" y="273" font-size="15">fresh</text><path class="arrow" d="M482 268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: </text><text x="880" y="308" font-size="16" text-anchor="end">this == &other is false</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">c = a;</text><text x="450" y="362" font-size="16" text-anchor="middle">c already exists, so copy assignment runs: it first builds a fresh copy of a's array</text></g> <g data-step="4"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 5</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="cell dead" x="520" y="204" width="60" height="36"/><text x="550" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="580" y="204" width="60" height="36"/><text x="610" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="640" y="204" width="60" height="36"/><text x="670" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="700" y="204" width="60" height="36"/><text x="730" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="760" y="204" width="60" height="36"/><text x="790" y="229" font-size="22" text-anchor="middle">0</text><text x="828" y="228" font-size="15">freed</text><path class="arrow" d="M298 242 H400 V222 H506"/><polygon class="head" points="520,222 506,215 506,229"/><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><text x="430" y="273" font-size="15">fresh</text><path class="arrow" d="M482 268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: </text><text x="880" y="308" font-size="16" text-anchor="end">c.data now dangles</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">delete[] data;</text><text x="450" y="362" font-size="16" text-anchor="middle">Only now is c's old 5-int array released; the fresh copy is already safe</text></g> <g data-step="5"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="hl" x="28" y="224" width="108" height="36"/><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="cell dead" x="520" y="204" width="60" height="36"/><text x="550" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="580" y="204" width="60" height="36"/><text x="610" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="640" y="204" width="60" height="36"/><text x="670" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="700" y="204" width="60" height="36"/><text x="730" y="229" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="760" y="204" width="60" height="36"/><text x="790" y="229" font-size="22" text-anchor="middle">0</text><text x="828" y="228" font-size="15">freed</text><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: </text><text x="880" y="308" font-size="16" text-anchor="end">c.data valid again</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">data = fresh; size = other.size;</text><text x="450" y="362" font-size="16" text-anchor="middle">c.data points to the fresh array and size is 3: the invariant holds again</text></g> <g data-step="6"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="130" width="68" height="44"/><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">99</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: </text><text x="880" y="308" font-size="16" text-anchor="end">only b's array changed</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">b.set(0, 99);</text><text x="450" y="362" font-size="16" text-anchor="middle">b writes into its own array: a and c are not affected</text></g> <g data-step="7"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="58" width="68" height="44"/><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="130" width="68" height="44"/><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">99</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="hl" x="516" y="246" width="68" height="44"/><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: 0 99 0</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << a.get(0) << ' ' << b.get(0) << ' ' << c.get(0);</text><text x="450" y="362" font-size="16" text-anchor="middle">Output: 0 99 0, so the three objects do not share any storage</text></g> <g data-step="8"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 create</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy constructor</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 copy assignment</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 independent</tspan></text><text x="20" y="48" font-size="15" opacity=".7">stack</text><text x="520" y="48" font-size="15" opacity=".7">free store</text><rect class="cell obj" x="20" y="56" width="290" height="64"/><text x="32" y="77" font-size="16" font-weight="700">a : IntArray</text><rect class="cell" x="32" y="84" width="100" height="28"/><text x="82" y="104" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="84" width="156" height="28"/><text x="200" y="104" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="62" width="60" height="36"/><text x="550" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="62" width="60" height="36"/><text x="610" y="87" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="62" width="60" height="36"/><text x="670" y="87" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 98 H400 V80 H506"/><polygon class="head" points="520,80 506,73 506,87"/><rect class="cell obj" x="20" y="128" width="290" height="64"/><text x="32" y="149" font-size="16" font-weight="700">b : IntArray</text><rect class="cell" x="32" y="156" width="100" height="28"/><text x="82" y="176" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="156" width="156" height="28"/><text x="200" y="176" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="134" width="60" height="36"/><text x="550" y="159" font-size="22" text-anchor="middle">99</text><rect class="cell heap" x="580" y="134" width="60" height="36"/><text x="610" y="159" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="134" width="60" height="36"/><text x="670" y="159" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 170 H400 V152 H506"/><polygon class="head" points="520,152 506,145 506,159"/><rect class="cell obj" x="20" y="200" width="290" height="64"/><text x="32" y="221" font-size="16" font-weight="700">c : IntArray</text><rect class="cell" x="32" y="228" width="100" height="28"/><text x="82" y="248" font-size="16" text-anchor="middle">size 3</text><rect class="cell" x="142" y="228" width="156" height="28"/><text x="200" y="248" font-size="16" text-anchor="middle">data</text><rect class="cell heap" x="520" y="250" width="60" height="36"/><text x="550" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="580" y="250" width="60" height="36"/><text x="610" y="275" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="640" y="250" width="60" height="36"/><text x="670" y="275" font-size="22" text-anchor="middle">0</text><path class="arrow" d="M298 242 H400 V268 H506"/><polygon class="head" points="520,268 506,261 506,275"/><text x="20" y="308" font-size="18">printed: 0 99 0</text><text x="880" y="308" font-size="16" text-anchor="end">3 objects, 3 arrays, 1 owner each</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // ~c(), ~b(), ~a() each run delete[] data;</text><text x="450" y="362" font-size="16" text-anchor="middle">At the closing brace each destructor deletes its own array exactly once</text></g> </svg> </figure>

## Let a vector own the memory · None of the Big Three to write ~~~cpp class Gradebook { public: Gradebook(const std::string& c, int n) : course(c), marks(n, 0) {} void set(int i, int m) { marks.at(i) = m; } int get(int i) const { return marks.at(i); } private: std::string course; std::vector<int> marks; // vector owns memory }; // Gradebook a("CS103", 3); Gradebook b = a; b.set(0, 99); → a: 0, b: 99 ~~~ **Takeaways:** `string` and `vector` members copy and release themselves, so `Gradebook` needs none of **the Big Three**. A class that needs one needs all three.

## The Gradebook example step by step <figure class="course-stepper" data-course-stepper> <svg class="course-array-address-figure course-l18zero-figure" viewBox="0 0 900 380" role="img" aria-label="A step-by-step trace of a class Gradebook that declares no destructor, copy constructor, or copy assignment, in 7 frames. Each object has two data members, course (a std::string) and marks (a std::vector of int that owns its elements). Frame 1: Gradebook a("CS103", 3) creates a with course CS103 and marks 0 0 0. Frame 2: Gradebook b = a runs the compiler-generated copy constructor, which copies course and marks member by member; b gets its own CS103 and its own 0 0 0. Frame 3: b.set(0, 99) changes only b, whose marks become 99 0 0, while a stays 0 0 0. Frame 4: the program prints 0 99. Frame 5: a = b runs the generated copy assignment, which assigns member by member, so a's marks become 99 0 0 and the vector manages its old storage itself. Frame 6: the program prints 99. Frame 7: at the closing brace the generated destructors run, b first and then a, and each vector releases its own elements; we wrote none of the Big Three." xmlns="http://www.w3.org/2000/svg"> <style> .course-l18zero-figure text{font-family:ui-monospace,Menlo,Consolas,monospace;fill:currentColor} .course-l18zero-figure .cell{fill:none;stroke:currentColor;stroke-width:2} .course-l18zero-figure .cell.obj{fill:rgba(142,232,216,.18)} .course-l18zero-figure .cell.heap{fill:rgba(255,214,102,.22)} .course-l18zero-figure .cell.dead{fill:rgba(255,120,120,.22);stroke-dasharray:6 5} .course-l18zero-figure .hl{fill:rgba(110,170,255,.35);stroke:none} .course-l18zero-figure .arrow{fill:none;stroke:currentColor;stroke-width:2.5} .course-l18zero-figure .head{fill:currentColor} </style> <g data-step="0"><text x="20" y="28" font-size="18"><tspan font-weight="700">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell obj" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell obj" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><text x="36" y="238" font-size="16">a owns its string and its vector</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="150" font-size="16">not created yet</text><text x="20" y="312" font-size="18">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">Gradebook a("CS103", 3);</text><text x="450" y="362" font-size="16" text-anchor="middle">The constructor sets course to "CS103" and marks to three zeros</text></g> <g data-step="1"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="hl" x="15" y="43" width="430" height="216"/><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell obj" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell obj" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="116" font-size="16">course</text><rect class="cell heap" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell heap" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="476" y="238" font-size="16">own copies of both members</text><text x="20" y="312" font-size="18">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">Gradebook b = a;</text><text x="450" y="362" font-size="16" text-anchor="middle">Generated copy constructor: string copies itself, vector copies its elements</text></g> <g data-step="2"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell obj" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell obj" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><rect class="hl" x="455" y="43" width="430" height="216"/><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="116" font-size="16">course</text><rect class="cell obj" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell heap" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell obj" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="476" y="238" font-size="16">only b changed; a is untouched</text><text x="20" y="312" font-size="18">printed: </text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">b.set(0, 99);</text><text x="450" y="362" font-size="16" text-anchor="middle">The two vectors are separate copies, so only b's first mark changes</text></g> <g data-step="3"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell obj" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell obj" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="116" font-size="16">course</text><rect class="cell obj" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell obj" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell obj" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="20" y="312" font-size="18">printed: 0 99</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << a.get(0) << ' ' << b.get(0);</text><text x="450" y="362" font-size="16" text-anchor="middle">Output: 0 99, proof that a and b do not share their marks</text></g> <g data-step="4"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="hl" x="15" y="43" width="430" height="216"/><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell heap" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell heap" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell heap" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell heap" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><text x="36" y="238" font-size="16">assigned member by member</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="116" font-size="16">course</text><rect class="cell obj" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell obj" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell obj" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="20" y="312" font-size="18">printed: 0 99</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">a = b;</text><text x="450" y="362" font-size="16" text-anchor="middle">Generated copy assignment: vector replaces a's old elements with a copy of b's</text></g> <g data-step="5"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">6 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">7 end</tspan></text><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook</text><text x="36" y="116" font-size="16">course</text><rect class="cell obj" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell obj" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell obj" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook</text><text x="476" y="116" font-size="16">course</text><rect class="cell obj" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell obj" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell obj" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell obj" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="20" y="312" font-size="18">printed: 0 99 and 99</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">std::cout << a.get(0);</text><text x="450" y="362" font-size="16" text-anchor="middle">Output: 99, a now holds its own copy of b's marks</text></g> <g data-step="6"><text x="20" y="28" font-size="18"><tspan opacity=".5">1 a</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">2 copy</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">3 set</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">4 print</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">5 assign</tspan><tspan opacity=".5"> · </tspan><tspan opacity=".5">6 print</tspan><tspan opacity=".5"> · </tspan><tspan font-weight="700">7 end</tspan></text><rect class="cell" x="20" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="36" y="74" font-size="18" font-weight="700">a : Gradebook (destroyed)</text><text x="36" y="116" font-size="16">course</text><rect class="cell dead" x="120" y="92" width="210" height="38"/><text x="225" y="119" font-size="20" text-anchor="middle">CS103</text><text x="36" y="170" font-size="16">marks</text><rect class="cell dead" x="120" y="144" width="70" height="40"/><text x="155" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell dead" x="190" y="144" width="70" height="40"/><text x="225" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="260" y="144" width="70" height="40"/><text x="295" y="173" font-size="22" text-anchor="middle">0</text><text x="36" y="214" font-size="16">vector owns its elements</text><text x="36" y="238" font-size="16">destroyed second</text><rect class="cell" x="460" y="48" width="420" height="206" style="stroke-dasharray:3 6"/><text x="476" y="74" font-size="18" font-weight="700">b : Gradebook (destroyed)</text><text x="476" y="116" font-size="16">course</text><rect class="cell dead" x="560" y="92" width="210" height="38"/><text x="665" y="119" font-size="20" text-anchor="middle">CS103</text><text x="476" y="170" font-size="16">marks</text><rect class="cell dead" x="560" y="144" width="70" height="40"/><text x="595" y="173" font-size="22" text-anchor="middle">99</text><rect class="cell dead" x="630" y="144" width="70" height="40"/><text x="665" y="173" font-size="22" text-anchor="middle">0</text><rect class="cell dead" x="700" y="144" width="70" height="40"/><text x="735" y="173" font-size="22" text-anchor="middle">0</text><text x="476" y="214" font-size="16">vector owns its elements</text><text x="476" y="238" font-size="16">destroyed first</text><text x="20" y="312" font-size="18">printed: 0 99 and 99</text><text x="450" y="336" font-size="18" text-anchor="middle" font-weight="700">} // end of main</text><text x="450" y="362" font-size="16" text-anchor="middle">Generated destructors run b then a; each vector frees its own elements</text></g> </svg> </figure>

## Common errors · One owner, one release | Error | Example | Result | |---|---|---| | No destructor | `new int[n]`, no `~IntArray()` | memory leak | | Destructor, shallow copy | `IntArray b = a;` | double deletion: undefined behavior | | Assignment keeps the old array | no `delete[] data;` | memory leak | | Copy parameter by value | `IntArray(IntArray other)` | compile error |

## Poll 2: Which lines call the copy constructor? <div class="course-poll-layout"> <div> ~~~cpp IntArray a(3); IntArray b = a; IntArray c(a); c = b; ~~~ **A)** line 2 only · **B)** lines 2 and 3<br>**C)** lines 2, 3, and 4 · **D)** line 4 only </div> <figure class="course-slido-qr"><img src="assets/slido-lecture18.svg" alt="QR code that opens the Lecture 18 Slido event." width="360" height="360"><figcaption>📊 sli.do · #4174771</figcaption></figure> </div>

## Questions about destructors and copying | Question | Answer | |---|---| | When does a destructor run? | At the end of the scope; at `delete` for a free-store object. | | Does pass-by-value copy? | Yes, with the copy constructor. Pass by `const&` (Lecture 8). | | A `unique_ptr` data member? | It cannot be copied (Lecture 14), so copying the class is a compile error. | | What does RAII stand for? | "Resource acquisition is initialization": lifetime controls the resource. |

## Summary: destructors, copying, and RAII - A **destructor** runs when the lifetime ends: last constructed, first destroyed. - **RAII:** the constructor acquires, the destructor releases. - The **copy constructor** creates; **copy assignment** replaces. The generated copy of a pointer is a **shallow copy**: two owners. - **The Big Three:** need one, write all three; with `vector` and `string` members, write none. **Reading:** Stroustrup, *PPP* (2nd ed.), “Vector and Free Store” and “Vectors and Arrays”; Savitch, “Friends, Overloaded Operators, and Arrays in Classes”.

## Lab practice: destructors and copying 1. Trace `Tag` objects in nested blocks and a loop; predict, then run. 1. Pass an `IntArray` by value; count the copies. Then pass by `const&`. 1. Write `class Text` that owns a `char` array: all three members. 1. Rewrite `Text` with `std::string` and delete the Big Three. [IC151 lab setup and batch schedule](ic151.html)