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std::initializer_list(3) C++ Standard Libary 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. A std::initializer_list object is automatically constructed when: * a braced-init-list is used to list-initialize an object, where the corresponding constructor accepts an std::initializer_list parameter * a braced-init-list is used as the right operand of assignment or as a function call argument, and the corresponding assignment operator/func- tion accepts an std::initializer_list parameter * a braced-init-list is bound to auto, including in a ranged for loop Initializer lists may be implemented as a pair of pointers or pointer and length. Copying a std::initializer_list does not copy the underlying ob- jects. The underlying array is a temporary array of type const T[N], in which each element is copy-initialized (except that narrowing conversions are invalid) from the corresponding element of the original initializer list. The lifetime of the underlying array is the same as any other temporary object, except that initializing an initializer_list object from the array extends the lifetime of the array exactly like binding a reference to a temporary (with the same exceptions, such as for initializing a non-static class member). The underlying array may be allocated in read-only memory. The program is ill-formed if an explicit or partial specialization of std::initializer_list is declared. Member types Member type Definition value_type T reference const T& const_reference const T& size_type std::size_t iterator const T* const_iterator const T* 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 under- lying array (C++17) (function template) Example // Run this code #include <iostream> #include <vector> #include <initializer_list> 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 re- turn statement // this is NOT a use of std::ini- tializer_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 size is now " << 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 de- duced 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 size is now 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior the lifetime of the underlying array CWG 1290 C++11 referenced by speci- fied same as other the initializer_list was not correctly tempo- rary objects specified CWG 1418 C++11 the type of the underlying array lacked const added const specializing initializer_list was LWG 2129 C++11 allowed made ill-formed but not guaranteed to work 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) http://cppreference.com 2022.07.31 std::initializer_list(3)
NAME | Synopsis | Member types | Member functions | Capacity | Iterators | Non-member functions | Example | Output: | See also
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