std::is_layout_compatible
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 |
[edit]Helper variable template
template<class T, class U > constexprbool 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 signedchar nor unsignedchar.
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 |
[edit]Example
#include <iomanip>#include <iostream>#include <type_traits> struct Foo {int x;char y;}; struct FooNua {int x;[[no_unique_address]]char y;}; class Bar {constint u =42;volatilechar v ='*';}; enum E0 :int{};enumclass E1 :int{}; static_assert ( std::is_layout_compatible_v<constvoid, volatilevoid>==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, unsignedlong>==false and std::is_layout_compatible_v<char*, constchar*>==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(){}
[edit]See also
(C++11) | checks if a type is a standard-layout type (class template) |
