std::initializer_list(3)

NAME
std::initializer_list - std::initializer_list

Synopsis
   Defined in header <initializer_list>
   template< class T > (since C++11)
   class initializer_list;
   An object of type std::initializer_list<T> is a lightweight proxy object that
   provides access to an array of objects of type const T (that may be allocated in
   read-only memory).
   A std::initializer_list object is automatically constructed when:
     * a brace-enclosed initializer list is used to list-initialize an object, where
       the corresponding constructor accepts an std::initializer_list parameter,
     * a brace-enclosed initializer list is used as the right operand of assignment or
       as a function call argument, and the corresponding assignment operator/function
       accepts an std::initializer_list parameter,
     * a brace-enclosed initializer list is bound to auto, including in a ranged for
       loop.
   std::initializer_list may be implemented as a pair of pointers or pointer and
   length. Copying a std::initializer_list does not copy the backing array of the
   corresponding initializer list.
   The program is ill-formed if an explicit or partial specialization of
   std::initializer_list is declared.

Member functions
   constructor creates an empty initializer list
                 (public member function)

Capacity
   size returns the number of elements in the initializer list
                 (public member function)

Iterators
   begin returns a pointer to the first element
                 (public member function)
   end returns a pointer to one past the last element
                 (public member function)

Non-member functions
   std::begin(std::initializer_list) overloads std::begin
   (C++11) (function template)
   std::end(std::initializer_list) specializes std::end
   (C++11) (function template)
         Free function templates overloaded for std::initializer_list
   rbegin returns a reverse iterator to the beginning of a
   crbegin container or array
   (C++14) (function template)
   rend returns a reverse end iterator for a container or
   crend array
   (C++14) (function template)
   empty checks whether the container is empty
   (C++17) (function template)
   data obtains the pointer to the underlying array
   (C++17) (function template)

Notes
     Feature-test macro Value Std Feature
   __cpp_initializer_lists 200806L (C++11) List-initialization and
                        std::initializer_list

Example
// Run this code
 #include <initializer_list>
 #include <iostream>
 #include <vector>
 template<class T>
 struct S
 {
     std::vector<T> v;
     S(std::initializer_list<T> l) : v(l)
     {
          std::cout << "constructed with a " << l.size() << "-element list\n";
     }
     void append(std::initializer_list<T> l)
     {
         v.insert(v.end(), l.begin(), l.end());
     }
     std::pair<const T*, std::size_t> c_arr() const
     {
         return {&v[0], v.size()}; // copy list-initialization in return statement
                        // this is NOT a use of std::initializer_list
     }
 };
 template<typename T>
 void templated_fn(T) {}
 int main()
 {
     S<int> s = {1, 2, 3, 4, 5}; // copy list-initialization
     s.append({6, 7, 8}); // list-initialization in function call
     std::cout << "The vector now has " << s.c_arr().second << " ints:\n";
     for (auto n : s.v)
         std::cout << n << ' ';
     std::cout << '\n';
     std::cout << "Range-for over brace-init-list: \n";
     for (int x : {-1, -2, -3}) // the rule for auto makes this ranged-for work
         std::cout << x << ' ';
     std::cout << '\n';
     auto al = {10, 11, 12}; // special rule for auto
     std::cout << "The list bound to auto has size() = " << al.size() << '\n';
 // templated_fn({1, 2, 3}); // compiler error! "{1, 2, 3}" is not an expression,
                        // it has no type, and so T cannot be deduced
     templated_fn<std::initializer_list<int>>({1, 2, 3}); // OK
     templated_fn<std::vector<int>>({1, 2, 3}); // also OK
 }

Output:
 constructed with a 5-element list
 The vector now has 8 ints:
 1 2 3 4 5 6 7 8
 Range-for over brace-init-list:
 -1 -2 -3
 The list bound to auto has size() = 3
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       std::initializer_list could have explicit the program is
   LWG 2129 C++11 specializations or partial ill-formed in this
                       specializations case

See also
   span a non-owning view over a contiguous sequence of objects
   (C++20) (class template)
   basic_string_view read-only string view
   (C++17) (class template)