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std::aligned_union

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.

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
{
    static constexpr std::size_t alignment_value = std::max({alignof(Types)...});
 
    struct type
    {
        alignas(alignment_value) char _s[std::max({Len, sizeof(Types)...})];
    };
};

Example

#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 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 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

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