copy ctor

[hooshdar3]

Hello.When are copy c-tors called?

Thanks in advance!

[Stewbond]

it's called if you ever pass a class into a function as an argument, or return a function;

class MyClass { };
MyClass A; // Default ctor
MyClass B(A); // Copy ctor 

MyClass SomeFunc(MyClass C) // Copy Ctor called to create C
{
   return C
}

cout << SomeFunc(A); // Copy ctor called to store the return value of SomeFunc before the cout

[wiskimo]

you could declare it as private so you'll see where it is called (by looking at compilation errors).

[JLBorges]

> When are copy c-tors called?

Whenever an object is copy initialized - ie. a new object is created which is a copy of another object of the same type.


> or return (from) a function;

Almost all compilers would apply RVO and elide the copy.

#include <iostream>

struct A
{
    A() { std::cout << "\tA::default_constructor\n\n" ; }
    A( const A& ) { std::cout << "\tA::copy_constructor\n\n" ; }
};

void foo( A ) {}

A bar() { A temp ; return temp ; }

int main()
{
    std::cout << "default-initialize an A\n" ; A a1 ; // default constructor of A

    std::cout << "make a copy of A\n" ; A a2 = a1 ; // copy constructor of A ;
    std::cout << "pass A by value\n" ; foo(a1) ; // copy constructor of A ;
    std::cout << "return A by value\n" ; A a3 = bar() ; // default constructor of A (NRVO)

    struct B { A a ; } ;
    std::cout << "default-initialize a B\n" ; B b1 ; // default constructor of A
    std::cout << "make a copy of B\n" ; B b2 = b1 ; // copy constructor of A
}

Output:

default-initialize an A
        A::default_constructor

make a copy of A
        A::copy_constructor

pass A by value
        A::copy_constructor

return A by value
        A::default_constructor

default-initialize a B
        A::default_constructor

make a copy of B
        A::copy_constructor

[hooshdar3]

wikipedia says it CAN be called upon return.Would you explain more about RVO?

[Catfish2]

http://en.wikipedia.org/wiki/Return_value_optimization

[hooshdar3]

I read it.the first example was complex