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std::is_aggregate

From cppreference.com
< cpp‎ | types
 
 
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Type properties
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is_aggregate
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Defined in header <type_traits>
template< class T >
struct is_aggregate;
(since C++17)

std::is_aggregate is a UnaryTypeTrait.

If T is an aggregate type, provides the member constant value equal true. For any other type, value is false.

If T is an incomplete type other than an array type or (possibly cv-qualified) void, the behavior is undefined.

If the program adds specializations for std::is_aggregate or std::is_aggregate_v, the behavior is undefined.

Contents

[edit] Template parameters

T - a type to check

[edit] 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>

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_is_aggregate 201703L (C++17) std::is_agregate

[edit] Example

#include <new>
#include <type_traits>
#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");
}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3823 C++17 The behavior is undefined if T is an array type but
std::remove_all_extents_t<T> is an incomplete type.
The behavior is defined regardless of the
incompleteness of std::remove_all_extents_t<T>
as long as T is an array type.