Tomilov Anatoliy
Tomilov Anatoliy

Reputation: 16691

Explicit defaulted default constructor and aggregates

How to explain the difference, when I compile #if 0 and #if 1 versions of the following code:

#include <cstdlib>

struct A
{ 
    explicit A() = default; // explicitly defaulted or deleted constructors are allowed for aggregates (since C++11)
#if 1
private :
#endif
    int i;
};

int
main()
{
    A a = {};
    return EXIT_SUCCESS;
}

What is the difference for expression A a = {}; depending on whether A is aggreagate or not?

Upvotes: 14

Views: 762

Answers (1)

Columbo
Columbo

Reputation: 60969

TL;DR: Clang and GCC are wrong in rejecting your code. CWG 1630´s resolution made default-initialization well-formed regardless of the chosen default constructor being explicit or not.


In the variation of your code in which i is private, A is not an aggregate, as these cannot have private members. As long as i is public, however, A is an aggregate1, and no constructor is invoked since aggregate initialization is performed (see blue box), so your constructor being explicit is irrelevant.

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However, as soon as you introduce the private member, you necessitate value-initialization as per the red box. Hence [dcl.init]/(8.2) applies:

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[dcl.init]/(7.1) defines default-initialization for this case:

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And §13.3.1.3 gives

For […] default-initialization, the candidate functions are all the constructors of the class of the object being initialized.

At no point is the original context - copy- or direct-initialization - considered. (§13.3.1.7 doesn't apply either.) In fact, this is intended; see CWG #1518:

This issue is resolved by the resolution of issue 1630: default initialization now uses 13.3.1.3 [over.match.ctor], which now permits explicit constructors for default-initialization.

Clang and GCC (and VC++) haven't implemented the corresponding DR yet and are thus incorrect in rejecting the code in C++14 mode.


1) Your class has a user-declared constructor, but it isn't user-provided, i.e. not impeding your class from being an aggregate. Recall the definition in [dcl.init.aggr]/1:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

Upvotes: 9

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