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This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-12-20

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 tovalue-initialize an object. However, the current rules make thefollowing 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 9.5.5 [dcl.init.list] paragraph 3 as follows:

List-initialization of an object or reference of typeT isdefined as follows:

• If the initializer list has no elements andT isa class type with a default constructor, the object isvalue-initialized.

• Otherwise, if the initializer list has no elementsandT is an aggregate, the initializer list is used toinitialize each of the members ofT. [Example:

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

—end example]

• IfOtherwise, ifT is anaggregate...

• ...

[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]