Question

A recent question got me wondering about explicit copy constructors. Here is a sample code that I tried compiling under Visual Studio 2005 :

struct A
{
    A() {}
    explicit A(const A &) {}
};

// #1 > Compilation error (expected behavior)
A retByValue()
{
    return A();
}

// #2 > Compiles just fine, but why ?
void passByValue(A a)
{
}

int main()
{
    A a;
    A b(a); // #3 > explicit copy construction : OK (expected behavior)
    A c = a; // #4 > implicit copy construction : KO (expected behavior)

    // Added after multiple comments : not an error according to VS 2005.
    passByValue(a);
    return 0;
}

Now for the questions :

• Is #2 allowed by the standard ? If it is, what is the relevant section describing this situation ?
• Do you known of any practical use for an explicit copy constructor ?

[EDIT] I just found a funny link on MSDN with the exact same situation, and a mysterious comment from the main function : “c is copied” (as if it was obvious). As pointed by Oli Charlesworth : gcc does not compile this code and I believe he’s right not to.

Answer 1

I believe the relevant sections of C++03 are §12.3.1 2:

An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax (8.5) or where casts (5.2.9, 5.4) are explicitly used. A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization (8.5).

and § 8.5 12:

The initialization that occurs in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), and brace-enclosed initializer lists (8.5.1) is called copy-initialization and is equivalent to the form

    T x = a;

The initialization that occurs in new expressions (5.3.4), static_cast expressions (5.2.9), functional notation type conversions (5.2.3), and base and member initializers (12.6.2) is called direct-initialization and is equivalent to the form

    T x(a);

Calling passByValue(a) involves copy-initialization, not direct-initialization, and thus should be an error, according to C++03 § 12.3.1 2.