Return fstream

Question

I have this function:

fstream open_user_file() const
{
    ...
}

but my compiler complains that fstreamcopy-constructor is implicitly deleted. Given that the compiler performs RVO, why is the copy constructor selected instead of the move constructor?

Otherwise, what is the best way to do this?

Answer 1

The accepted answer is currently invalid.

When returning a local variable with automatic storage of the same type as the declared return type of the function, a two-phase process occurs:

fstream open_user_file() const
{
    fstream f;

    /*...*/

    return f;
}

• Choosing a constructor for the copy is first performed as if the object was designated rvalue.

• , rvalue (, cv), , lvalue.

, f , (, , ) f. Else, f , (, , ) f. f .

, :

return std::move(f);

, . fstream :

return f;

. f , :

return std::move(f);

. , RVO.

gcc 4.8 ( ). . ++ 98, ++ 03 gcc 4.8 . ++ 11 .

Answer 2

return, . .

fstream open_user_file() const
{
    fstream f;

    /*...*/

    return std::move(f);
}

/ , , / / .

...

, :

, , object - , lvalue, ,, rvalue. rvalue (, cv-), , . [. , . , elision , , .