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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.

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.

Contents

[edit] 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>

[edit] Notes

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

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

[edit] Example

#include <type_traits>
#include <iostream>
 
struct Foo {
    int x;
    char y;
};
 
class Bar {
    const int u = 42;
    volatile char v = '*';
};
 
enum E0 : int {};
enum class E1 : int {};
 
int main()
{
    std::cout << std::boolalpha
        << std::is_layout_compatible_v<const void, volatile void> << '\n'
        << std::is_layout_compatible_v<Foo, Bar> << '\n'
        << std::is_layout_compatible_v<Foo[2], Bar[2]> << '\n'
        << std::is_layout_compatible_v<int, E0> << '\n'
        << std::is_layout_compatible_v<E0, E1> << '\n'
        << std::is_layout_compatible_v<long, unsigned long> << '\n'
        << std::is_layout_compatible_v<char*, const char*> << '\n'
        << std::is_layout_compatible_v<char*, char* const> << '\n';
}

Output:

true
true
false
false
true
false
false
true

[edit] See also

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