C++ – Copy-Initialization and Copy Constructor Call

c++g++language-lawyer

class AAA {
public:
    explicit AAA(const AAA&) {}
    AAA(int) {}
};


int main() {
    AAA a = 1;
    return 0;
}

In the above code, as I understand, though elided in most cases, the copy constructor is still semantically required to be called. My question is, is the call explicit or implicit? For a long time I have the conclusion in my mind that the call to AAA::AAA(int) is implicit but the call to the copy constructor is not. Today I accidentally got g++ to compile the above code and it reported error. (VC12 compiles OK.)

In section 8.5 of the standard:

If the destination type is a (possibly cv-qualified) class type:

  • If the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source
    type is the same class as, or a derived class of, the class of the
    destination, constructors are considered. The applicable constructors
    are enumerated (13.3.1.3), and the best one is chosen through overload
    resolution (13.3). The constructor so selected is called to initialize
    the object, with the initializer expression or expression-list as its
    argument(s). If no constructor applies, or the overload resolution is
    ambiguous, the initialization is ill-formed.

  • Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversion sequences that can convert from the source
    type to the destination type or (when a conversion function is used)
    to a derived class thereof are enumerated as described in 13.3.1.4,
    and the best one is chosen through overload resolution (13.3). If the
    conversion cannot be done or is ambiguous, the initialization is
    ill-formed. The function selected is called with the initializer
    expression as its argument; if the function is a constructor, the call
    initializes a temporary of the cv-unqualified version of the
    destination type. The temporary is a prvalue. The result of the call
    (which is the temporary for the constructor case) is then used to
    direct-initialize, according to the rules above, the object that is
    the destination of the copy-initialization. In certain cases, an
    implementation is permitted to eliminate the copying inherent in this
    direct-initialization by constructing the intermediate result directly
    into the object being initialized; see 12.2, 12.8.

The bolded direct-initialize in the above quotes means the call to copy constructor is explicit, right? Is g++ wrong or my interpretation of the standard wrong?

Best Answer

Looks like this bug: g++ fails to call explicit constructors in the second step of copy initialization

g++ fails to compile the following code

struct X
{
    X(int) {}
    explicit X(X const &) {}
};

int main()
{
    X x = 1; // error: no matching function for call to 'X::X(X)'
}

The second step of a copy initialization (see 8.5/16/6/2) is a direct-initialization where explicit constructors shall be considered as candidate functions.

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