# 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 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.

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
    {
        static constexpr std::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 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

### 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)

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