C++11 default class member initialization with initializer list , simultaneously

Question

Can someone point me please, to a corresponding paragraph of the C++ standard, or maybe can provide some explanation why my code does not compile if I uncomment the text ({123})?

Generally speaking I understand what something wrong with usage of default member initialization and initialization via initializer list, but I can't refer to exact reasons.

enum class MY: int
{
    A = 1
};

struct abc
{
    int a;/*{123};*/  //compilation failed if uncommented
    MY m;
};

abc a = {1, MY::A};

Compiler error, in case of uncommented text:

error: could not convert ‘{1, A}’ from ‘<brace-enclosed initializer list>’ to ‘abc’

Answer 1

The below syntax:

abc a = {1, MY::A};

is a list-initialization which may perform differently depending on the type being initialized. Without the non-static data member initializer (/*{123};*/), your struct is an aggregate and the case falls under [dcl.init.list]/p3:

• Otherwise, if T is an aggregate, aggregate initialization is performed.

However, to be an aggregate type, the following conditions must be met in C++11:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no brace-or-equal-initializers for non-static data members (9.2), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

That is, the usage of NSDMI (non-static data member initialization) breaks the above set of rules, and as a result, an instance of this type can no longer be list-initialized.

This rule has changed in C++14, and the current wording reads [dcl.init.aggr]/p1:

An aggregate is an array or a class with

• (1.1) no user-provided, explicit, or inherited constructors ([class.ctor]),

• (1.2) no private or protected non-static data members ([class.access]),

• (1.3) no virtual functions, and

• (1.4) no virtual, private, or protected base classes ([class.mi]).

[ Note: Aggregate initialization does not allow accessing protected and private base class' members or constructors. — end note ]