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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.

If T or U is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, 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.

If the program adds specializations for std::is_layout_compatible or std::is_layout_compatible_v, the behavior is undefined.

Contents

Helper variable template

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

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 Feature
__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

(C++11)
 checks if a type is a standard-layout type
(class template) [edit]
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