This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

990. Value initialization with multiple initializer-list constructors

[Voted into WP at March, 2010 meeting as part of document N3079.]

It should always be possible to use the new brace syntax to value-initialize an object. However, the current rules make the following example ill-formed because of ambiguity:

    struct S {
      S();
      S(std::initializer_list<int>);
      S(std::initializer_list<double>);
    };
    S s{};    // Ambiguous initializer-list constructor reference,
              // not value initialization.

Proposed resolution (February, 2010):

Change 9.4.5 [dcl.init.list] paragraph 3 as follows:

List-initialization of an object or reference of type T is defined as follows:

• If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

• Otherwise, if the initializer list has no elements and T is an aggregate, the initializer list is used to initialize each of the members of T. [Example:

  struct A {
    A(std::initializer_list<int>);  // #1
  };
  struct B {
    A a;
  };
  B b { };    // OK, uses #1
  B b { 1 };  // error

—end example]

• If Otherwise, if T is an aggregate...

• ...

[Example:

  struct S {
    S(std::initializer_list<double>);  // #1
    S(std::initializer_list<int>);     // #2
    S();                                     // #3
    // ...
  };
  S s1 = { 1.0, 2.0, 3.0 };            // invoke #1
  S s2 = { 1, 2, 3 };                  // invoke #2
  S s3 = { };                          // invoke #3 (for value-initialization; see above)

—end example]