Question

I have the following sample code (a stripped down version of my programme)

Class ‘some_class’ has a constructor with default parameters. The compiler is able to recognise this constructor as a copy constructor. In the main function, this constructor is called when I order for a copy constructed object called ‘b’. But when I construct ‘c’ from a function result, the compiler calls a compiler generated copy constructor (which copies the bit pattern). I can tell by the value of c.some_data, which should have been set by my own copy constructor to a value of 2.

1) What does the standard say about this?
2) Is my compiler broken?

I use MinGW with no options but a specification of my source file name and a name for the executable. I got my port of the gnu open source compiler from the official MinGW website, I’m using the latest version. Have I found a bug, or is this due to my (mis)understanding of c++?

Thanks in advance

#include <iostream>
#include <string>

class some_class
{
public:
    some_class(int p = 0) :
        some_data(p)
    {
        std::cout << "user defined constructor (p = " << p << ")" << std::endl;
    }
    some_class(const some_class &, int = 0)
    {
        std::cout << "user defined copy constructor" << std::endl;

        some_data = 2;
    }

    int some_data;
};

extern some_class some_function_returning_some_class_object();

int main(int, char **)
{
        std::cout << "creating a, with no parameters" << std::endl;
    some_class a;
        std::cout << "creating b, a copy of a" << std::endl;
    some_class b = a;
        std::cout << "creating c, copy constructed from a function result" << std::endl;
    some_class c = some_function_returning_some_class_object();
        std::cout << "c.some_data = " << c.some_data << std::endl;
}

some_class some_function_returning_some_class_object()
{
    some_class a(1);

    return a;
}

The output is as follows:

creating a, with no parameters
user defined constructor (p = 0)
creating b, a copy of a
user defined copy constructor
creating c, copy constructed from a function result
user defined constructor (p = 1)
c.some_data = 1

Answer 1

The compiler is not using the compiler-defined default copy constructor. It is presumably using return value optimization to skip the copy altogether.