std::is_layout_compatible - cppreference.com (original) (raw)
| | | | | ------------------------------------------------------------ | | ------------- | | 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 > constexpr bool is_layout_compatible_v = is_layout_compatible<T, U>::value; | | (since C++20) | | --------------------------------------------------------------------------------------------------------------- | | ------------- |
Inherited from std::integral_constant
Member constants
| | true if T and U are layout-compatible, false otherwise (public static member constant) | | ----------------------------------------------------------------------------------------- |
Member functions
| | converts the object to bool, returns value (public member function) | | ---------------------------------------------------------------------- | | | returns value (public member function) |
Member types
[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.
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 #include #include 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() {}