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.
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.
The behavior of a program that adds specializations for is_layout_compatible
or is_layout_compatible_v
is undefined.
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> |
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 testing macro: __cpp_lib_is_layout_compatible
.
Example
#include <type_traits> #include <iomanip> #include <iostream> struct Foo { int x; char y; }; class Bar { const int u = 42; volatile char v = '*'; }; enum E0 : int {}; enum class E1 : int {}; #define SHOW(...) std::cout << std::setw(54) << #__VA_ARGS__ << " = " << __VA_ARGS__ << '\n' int main() { std::cout << std::boolalpha << std::left; SHOW(std::is_layout_compatible_v<const void, volatile void>); SHOW(std::is_layout_compatible_v<Foo, Bar>); SHOW(std::is_layout_compatible_v<Foo[2], Bar[2]>); SHOW(std::is_layout_compatible_v<int, E0>); SHOW(std::is_layout_compatible_v<E0, E1>); SHOW(std::is_layout_compatible_v<long, unsigned long>); SHOW(std::is_layout_compatible_v<char*, const char*>); SHOW(std::is_layout_compatible_v<char*, char* const>); }
Output:
std::is_layout_compatible_v<const void, volatile void> = true std::is_layout_compatible_v<Foo, Bar> = true std::is_layout_compatible_v<Foo[2], Bar[2]> = false std::is_layout_compatible_v<int, E0> = false std::is_layout_compatible_v<E0, E1> = true std::is_layout_compatible_v<long, unsigned long> = false std::is_layout_compatible_v<char*, const char*> = false std::is_layout_compatible_v<char*, char* const> = true
See also
(C++11)
|
checks if a type is a standard-layout type (class template) |
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