< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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 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 a trivial and standard-layout 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)
(deprecated in C++23)

[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

#include <type_traits>
#include <iostream>
#include <string>
int main()
    << 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>;
    << "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';

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

[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]
(C++11)(deprecated in C++23)
defines the type suitable for use as uninitialized storage for types of given size
(class template) [edit]