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(C++11)(until C++20)(C++17)
Defined in header <type_traits>
template< std::size_t Len, class... Types >
struct aligned_union;
(since C++11)

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 supported.

The behavior of a program that adds specializations for aligned_union is undefined.


[edit] Member types

Name Definition
type the trivial type suitable for storage of any type from Types

[edit] Helper types

template< std::size_t Len, class... Types >
using aligned_union_t = typename aligned_union<Len,Types...>::type;
(since C++14)

[edit] Member constants

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

[edit] Possible implementation

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

[edit] Example

[edit] 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 wasn't required requires complete types

[edit] See also

obtains the type's alignment requirements
(class template) [edit]
defines the type suitable for use as uninitialized storage for types of given size
(class template) [edit]