# std::aligned_union

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< cpp‎ | types

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.

### Member types

Name | Definition
`type` | the trivial 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)

### 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

| This section is incomplete Reason: no example

### 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

### See also

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

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