21 Language support library [language.support]

21.9 Initializer lists [support.initlist]

The header <initializer_­list> defines a class template and several support functions related to list-initialization (see [dcl.init.list]).
All functions specified in this subclause are signal-safe.

21.9.1 Header <initializer_­list> synopsis [initializer_list.syn]

namespace std {
  template<class E> class initializer_list {
  public:
    using value_type      = E;
    using reference       = const E&;
    using const_reference = const E&;
    using size_type       = size_t;

    using iterator        = const E*;
    using const_iterator  = const E*;

    constexpr initializer_list() noexcept;

    constexpr size_t size() const noexcept;     // number of elements
    constexpr const E* begin() const noexcept;  // first element
    constexpr const E* end() const noexcept;    // one past the last element
  };

  // [support.initlist.range], initializer list range access
  template<class E> constexpr const E* begin(initializer_list<E> il) noexcept;
  template<class E> constexpr const E* end(initializer_list<E> il) noexcept;
}
An object of type initializer_­list<E> provides access to an array of objects of type const E.
[Note
:
A pair of pointers or a pointer plus a length would be obvious representations for initializer_­list.
initializer_­list is used to implement initializer lists as specified in [dcl.init.list].
Copying an initializer list does not copy the underlying elements.
end note
]
If an explicit specialization or partial specialization of initializer_­list is declared, the program is ill-formed.

21.9.2 Initializer list constructors [support.initlist.cons]

constexpr initializer_list() noexcept;
Effects: Constructs an empty initializer_­list object.
Postconditions: size() == 0.

21.9.3 Initializer list access [support.initlist.access]

constexpr const E* begin() const noexcept;
Returns: A pointer to the beginning of the array.
If size() == 0 the values of begin() and end() are unspecified but they shall be identical.
constexpr const E* end() const noexcept;
Returns: begin() + size().
constexpr size_t size() const noexcept;
Returns: The number of elements in the array.
Complexity: Constant time.

21.9.4 Initializer list range access [support.initlist.range]

template<class E> constexpr const E* begin(initializer_list<E> il) noexcept;
Returns: il.begin().
template<class E> constexpr const E* end(initializer_list<E> il) noexcept;
Returns: il.end().