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std::aligned_union(3) C++ Standard Libary std::aligned_union(3) NAME std::aligned_union - std::aligned_union Synopsis Defined in header <type_traits> template< std::size_t Len, class... Types > (since C++11) struct aligned_union; (deprecated in C++23) Provides the nested type type, which is a trivial standard-layout type of a size and alignment suitable for use as uninitialized storage for an object of any of the types listed in Types. The size of the storage is at least Len. std::aligned_union also determines the strictest (largest) alignment requirement among all Types and makes it available as the constant alignment_value. If sizeof...(Types) == 0 or if any of the types in Types is not a complete object type, the behavior is undefined. It is implementation-defined whether any extended alignment is sup- ported. The behavior of a program that adds specializations for aligned_union is undefined. Member types Name Definition type a trivial and standard-layout type suitable for storage of any type from Types Helper types template< std::size_t Len, class... Types > (since C++14) using aligned_union_t = typename (dep- recated in C++23) aligned_union<Len,Types...>::type; Member constants alignment_value the strictest alignment requirement of all Types [static] (public static member constant) Possible implementation #include <algorithm> template <std::size_t Len, class... Types> struct aligned_union { static constexpr std::size_t alignment_value = std::max({alig- nof(Types)...}); struct type { alignas(alignment_value) char _s[std::max({Len, sizeof(Types)...})]; }; }; Example // Run this code #include <type_traits> #include <iostream> #include <string> int main() { std::cout << sizeof(std::aligned_union_t<0, char>) << ' ' // 1 << sizeof(std::aligned_union_t<2, char>) << ' ' // 2 << sizeof(std::aligned_union_t<2, char[3]>) << ' ' // 3 (!) << sizeof(std::aligned_union_t<3, char[4]>) << ' ' // 4 << sizeof(std::aligned_union_t<1, char, int, double>) << ' ' // 8 << sizeof(std::aligned_union_t<12, char, int, double>) << '\n'; // 16 (!) using var_t = std::aligned_union<16, int, std::string>; std::cout << "var_t::alignment_value = " << var_t::alignment_value << '\n' << "sizeof(var_t::type) = " << sizeof(var_t::type) << '\n'; var_t::type aligned_storage; int* int_ptr = new(&aligned_storage) int(42); // placement new std::cout << "*int_ptr = " << *int_ptr << '\n'; std::string* string_ptr = new(&aligned_storage) std::string("bar"); std::cout << "*string_ptr = " << *string_ptr << '\n'; *string_ptr = "baz"; std::cout << "*string_ptr = " << *string_ptr << '\n'; string_ptr->~basic_string(); } Possible output: 1 2 3 4 8 16 var_t::alignment_value = 8 sizeof(var_t::type) = 32 *int_ptr = 42 *string_ptr = bar *string_ptr = baz Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behav- ior LWG 2979 C++11 complete type wasn't required requires complete types See also alignment_of obtains the type's alignment require- ments (C++11) (class template) aligned_storage defines the type suitable for use as uninitialized (C++11)(deprecated in C++23) storage for types of given size (class template) http://cppreference.com 2022.07.31 std::aligned_union(3)
NAME | Synopsis | Member types | Helper types | Member constants | Possible implementation | Example | Possible output: | See also
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