# std::is_layout_compatible

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Defined in header `<type_traits>` |  |
template< class T, class U > struct is_layout_compatible; |  | (since C++20)

If `T` and `U` are layout-compatible types, provides the member constant `value` equal to true. Otherwise `value` is false.

Every type is layout-compatible with its any cv-qualified versions, even if it is not an object type.

`T` and `U` shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

The behavior of a program that adds specializations for `std::is_layout_compatible` or `std::is_layout_compatible_v` is undefined.

### Helper variable template

template< class T, class U > inline constexpr bool is_layout_compatible_v = is_layout_compatible<T, U>::value; |  | (since C++20)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` and `U` are layout-compatible, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Notes

A signed integer type and its unsigned counterpart are not layout-compatible. char is layout-compatible with neither signed char nor unsigned char.

Similar types are not layout-compatible if they are not the same type after ignoring top-level cv-qualification.

An enumeration type and its underlying type are not layout-compatible.

Array types of layout-compatible but different element types (ignoring cv-qualification) are not layout-compatible, even if they are of equal length.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_is_layout_compatible` | 201907L | (C++20) | `std::is_layout_compatible`

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <type_traits>

    struct Foo
    {
        int x;
        char y;
    };

    struct FooNua
    {
        int x;
        [[no_unique_address]] char y;
    };

    class Bar
    {
        const int u = 42;
        volatile char v = '*';
    };

    enum E0 : int {};
    enum class E1 : int {};

    static_assert
    (
        std::is_layout_compatible_v<const void, volatile void> == true  and
        std::is_layout_compatible_v<Foo, Bar>                  == true  and
        std::is_layout_compatible_v<Foo[2], Bar[2]>            == false and
        std::is_layout_compatible_v<int, E0>                   == false and
        std::is_layout_compatible_v<E0, E1>                    == true  and
        std::is_layout_compatible_v<long, unsigned long>       == false and
        std::is_layout_compatible_v<char*, const char*>        == false and
        std::is_layout_compatible_v<char*, char* const>        == true  and
        std::is_layout_compatible_v<Foo, FooNua>               == false // Note [1]
    );

    // [1] MSVC erroneously fails this assert

    int main() {}

### See also

is_standard_layout (C++11) | checks if a type is a standard-layout type (class template)

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