std::aligned_union

 

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Defined in header <type_traits>
template<std::size_t Len, class... Types> struct aligned_union;		(since C++11) (deprecated in C++23)

Provides the nested type type, which is a trivialstandard-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 supported.

If the program adds specializations for std::aligned_union, the behavior 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> using aligned_union_t =typename aligned_union<Len,Types...>::type;		(since C++14) (deprecated in C++23)

Member constants

alignment_value [static]

the strictest alignment requirement of all Types (public static member constant)

Possible implementation

#include <algorithm>   template<std::size_t Len, class... Types>struct aligned_union {staticconstexprstd::size_t alignment_value =std::max({alignof(Types)...});   struct type { alignas(alignment_value)char _s[std::max({Len, sizeof(Types)...})];};};

Example

Run this code

#include <iostream>#include <string>#include <type_traits>   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 newstd::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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2979	C++11	complete type was not required	requires complete types

See also

alignment_of (C++11)	obtains the type's alignment requirements (class template)
aligned_storage (since C++11)(deprecated in C++23)	defines the type suitable for use as uninitialized storage for types of given size (class template)