NAME¶

std::is_layout_compatible - std::is_layout_compatible

Synopsis¶

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

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.

Member constants¶

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

Member functions¶

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

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