Introducing a name for brace-or-equal-initializers for non-static data members

class X {
  int a = 123;
  X() {} // initialize 'a' to 123
};

• Non-static data member initializers: This is the name of the papers introducing the feature. It is also the name used by GCC, where it is frequently shortened to "NSDMI" due to its unwieldy length.
• brace-or-equal-initializer for a non-static data member: This is the name used by the standard.
• In-class initializers
• Default member initializers

Based on many discussions within and outside the committee, this paper proposes that we use the term default member initializer for this construct. This term is specific enough to uniquely identify the scenario, reflects the symmetry between this construct and similar cases (default arguments and default template arguments, and a constructor's mem-initializers), and is terse enough that an abbreviation (such as NSDMI) does not need to be invented.

Proposed wording

Otherwise, if a member-declarator declares a non-static data member (9.2) of a class X, the expression this is a prvalue of type "pointer to X" within the optional brace-or-equal-initializerdefault member initializer. It shall not appear elsewhere in the member-declarator.

class Outer {
    int a[sizeof(*this)];            // error: not inside a member function
    unsigned int sz = sizeof(*this); // OK: in brace-or-equal-initializerdefault member initializer
[...]

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from its brace-or-equal-initializerdefault member initializer (9.2) or, if there is no brace-or-equal-initializerdefault member initializer, from an empty initializer list (8.5.4).

If a reference member is initialized from its brace-or-equal-initializerdefault member initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that brace-or-equal-initializerdefault member initializer, the program is ill-formed.

Within the class member-specification, the class is regarded as complete within function bodies, default arguments, using-declarations introducing inheriting constructors (12.9), exception-specifications, and brace-or-equal-initializers for non-static data membersdefault member initializers (including such things in nested classes)

A brace-or-equal-initializer shall appear only in the declaration of a data member. (For static data members, see 9.4.2; for non-static data members, see 12.6.2 and 8.5.1). A brace-or-equal-initializer for a non-static data member specifies a default member initializer for the member, and shall not directly or indirectly cause the implicit definition of a defaulted default constructor for the enclosing class or the exception specification of that constructor.

A union-like class is a union or a class that has an anonymous union as a direct member. A union-like class X has a set of variant members. If X is a union, a non-static data member of X that is not an anonymous union is a variant member of X. In addition, a non-static data member of an anonymous union that is a member of X is also a variant member of X. At most one variant member of a union may have a brace-or-equal-initializerdefault member initializer.

• any non-static data member with no brace-or-equal-initializerdefault member initializer is of reference type,

• no non-static data member of its class has a brace-or-equal-initializerdefault member initializer, and

• if the entity is a non-static data member that has a brace-or-equal-initializerdefault member initializer (9.2) and either
  • the constructor's class is a union (9.5), and no other variant member of that union is designated by a mem-initializer-id or the constructor's class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id,
  the entity is initialized from its default member initializer as specified in 8.5;

If a given non-static data member has both a brace-or-equal-initializerdefault member initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member's brace-or-equal-initializerdefault member initializer is ignored. [ Example: Given

  struct A {
  int i = /* some integer expression with side effects */ ; A(int arg) : i(arg) { }
  // ...
  };

the A(int) constructor will simply initialize i to the value of arg, and the side effects in i's brace-or-equal-initializerdefault member initializer will not take place. — end example ]

A temporary expression bound to a reference member from a brace-or-equal-initializerdefault member initializer is ill-formed.

[…] When typeid is used in a constructor (including the mem-initializer or brace-or-equal-initializerdefault member initializer for a non-static data member) or in a destructor, […]

[…] When a dynamic_cast is used in a constructor (including the mem-initializer or brace-or-equal-initializerdefault member initializer for a non-static data member) or in a destructor, […]

The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members. [ Note: brace-or-equal-initializerdefault member initializers of non-static data members are ignored. See also the example in 12.6.2. — end note ]