std::is_aggregate

 

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Type categories is_void (C++11) is_null_pointer (C++11)(DR*) is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_reference (C++11)  is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11)   is_fundamental (C++11) is_arithmetic (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) Type properties is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_implicit_lifetime (C++23) is_trivial (C++11)(deprecated in C++26) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(until C++20*) is_pod (C++11)(deprecated in C++20) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) Type trait constants integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)	Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20)   is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) reference_converts_from_temporary (C++23) is_base_of (C++11) is_virtual_base_of (C++26) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) Type modifications remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) Type transformations aligned_storage (C++11)(deprecated in C++23) aligned_union (C++11)(deprecated in C++23) decay (C++11) remove_cvref (C++20) result_ofinvoke_result (C++11)(until C++20*)(C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) type_identity (C++20) enable_if (C++11) void_t (C++17)
Compile-time rational arithmetic
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integer_sequence (C++14)

 

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.

Template parameters

T

-

a type to check

Helper variable template

template<class T > constexprbool 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-test macro	Value	Std	Feature
__cpp_lib_is_aggregate	201703L	(C++17)	std::is_agregate

Example

Run this code

#include <algorithm>#include <cassert>#include <cstddef>#include <new>#include <string_view>#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){ifconstexpr(std::is_aggregate_v<T>)return::new(static_cast<void*>(p)) T{std::forward<Args>(args)...};elsereturn::new(static_cast<void*>(p)) T(std::forward<Args>(args)...);}   struct A {int x, y;}; static_assert(std::is_aggregate_v<A>);   struct B {int i;std::string_view str;   B(int i, std::string_view str): i(i), str(str){}}; static_assert(not std::is_aggregate_v<B>);   template<typename... Ts>using aligned_storage_t = alignas(Ts...)std::byte[std::max({sizeof(Ts)...})];   int main(){ aligned_storage_t<A, B> storage;   A& a =*construct(reinterpret_cast<A*>(&storage), 1, 2);assert(a.x==1 and a.y==2);   B& b =*construct(reinterpret_cast<B*>(&storage), 3, "4");assert(b.i==3 and b.str=="4");}

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.