std::is_aggregate
Defined in header <type_traits> |
||
---|---|---|
template< class T > struct is_aggregate; |
(since C++17) |
Checks if T
is an aggregate type. The member constant value
is equal to true
if T
is an aggregate type and false
otherwise.
The behavior is undefined if std::remove_all_extents_t<T>
is an incomplete type other than (possibly cv-qualified) void
.
The behavior of a program that adds specializations for is_aggregate
or is_aggregate_v
is undefined.
Template parameters
T | - | a type to check |
Helper variable template
template< class T > inline constexpr bool is_aggregate_v = is_aggregate<T>::value; |
(since C++17) |
Inherited from std::integral_constant
Member constants
value
[static]
|
true if T is an aggregate type , 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
Feature testing macro: __cpp_lib_is_aggregate
.
Example
#include <type_traits> #include <new> #include <utility> // constructs a T at the uninitialized memory pointed to by p // using list-initialization for aggregates and non-list initialization otherwise template<class T, class... Args> T* construct(T* p, Args&&... args) { if constexpr(std::is_aggregate_v<T>) { return ::new (static_cast<void*>(p)) T{std::forward<Args>(args)...}; } else { return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...); } } struct A { int x, y; }; struct B { B(int, const char*) { } }; int main() { std::aligned_union_t<1, A, B> storage; [[maybe_unused]] A* a = construct(reinterpret_cast<A*>(&storage), 1, 2); [[maybe_unused]] B* b = construct(reinterpret_cast<B*>(&storage), 1, "hello"); }
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