Copy constructor not called when initializing an object with return value of a function

Question

Consider the following code:

#include 

using namespace std;

class A
{
    public:
        int a;
        A(): a(5)
        {
           cout &l         


        

Answer 1

#include <iostream>

using namespace std;

class A
{
public:
    int a;
    A(): a(5)
    {
        cout << "Constructing: " << (void *)this << std::endl;
    }
    A(const A &b)
    {
        a = b.a;
        cout << "Copy Constructor: " << (void *)this << " from " << (void *)&b << std::endl;
    }
    A fun(A a)
    {
        return a;
    }
};

int main()
{

    A a, c;
    A b = a.fun(c);

    std::cout << "a:" << (void *)&a << std::endl <<
              "b:" << (void *)&b << std::endl <<
              "c:" << (void *)&c << std::endl;
    return 0;
}

Yields:

Constructing: 0x7fffbb377220
Constructing: 0x7fffbb377210
Copy Constructor: 0x7fffbb377230 from 0x7fffbb377210
Copy Constructor: 0x7fffbb377200 from 0x7fffbb377230
a:0x7fffbb377220
b:0x7fffbb377200
c:0x7fffbb377210

So it constructs a, constructs c, copies c to an intermediate (argument a of the function), and then copies the intermediate directly into b, skipping the typical copying of a to a return intermediate. This is even better demonstrated if you pass by value (change to A fun(const A& a):

Constructing: 0x7fff8e9642b0
Constructing: 0x7fff8e9642a0
Copy Constructor: 0x7fff8e964290 from 0x7fff8e9642a0
a:0x7fff8e9642b0
b:0x7fff8e964290
c:0x7fff8e9642a0

a is constructed, c is constructed, c is copied directly to b, despite b not being passed to fun!

Answer 2

The copy that is elided is the copy of the temporary return value into b. Without elision the return value is initialized from a and copied to b. Instead, the temporary that would otherwise hold the return value is constructed into b and initialized with a. [class.copy]/31:

when a temporary class object that has not been bound to a reference (12.2) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

You can observe this if you add an additional output in fun:

A fun(A a)
{
    cout << "fun!" << endl;
    return a;
}

Then with the elision you'll get

[…]
fun!
Copy Constructor

And without:

[…]
fun!
Copy Constructor
Copy Constructor