std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible

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Feature test macros(C++20)
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Concepts library(C++20)
Diagnostics library
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Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
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Text processing library
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Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
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Symbols index
External libraries

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Metaprogramming library

Type traits

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

(C++11)

(C++11)

(C++11)

(C++11)

(C++14)

(C++11)

(C++17)

(C++23)

(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)

(C++11)

(C++11)

(C++20)

(C++20)

(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

Compile-time integer sequences

integer_sequence

(C++14)

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Defined in header`<type_traits>`
template<class T> struct is_default_constructible;	(1)	(since C++11)
template<class T> struct is_trivially_default_constructible;	(2)	(since C++11)
template<class T> struct is_nothrow_default_constructible;	(3)	(since C++11)

1) Provides the member constantvalue equal tostd::is_constructible<T>::value.

2) Provides the member constantvalue equal tostd::is_trivially_constructible<T>::value.

3) Provides the member constantvalue equal tostd::is_nothrow_constructible<T>::value.

IfT 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 any of the templates described on this page, the behavior is undefined.

[edit]Helper variable templates

template<class T> inlineconstexprbool is_default_constructible_v= is_default_constructible<T>::value;		(since C++17)
template<class T> inlineconstexprbool is_trivially_default_constructible_v= is_trivially_default_constructible<T>::value;		(since C++17)
template<class T> inlineconstexprbool is_nothrow_default_constructible_v= is_nothrow_default_constructible<T>::value;		(since C++17)

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is default-constructible,false otherwise
(public static member constant)

Member functions

operator bool	converts the object tobool, returnsvalue (public member function)
operator() (C++14)	returnsvalue (public member function)

Member types

Type	Definition
`value_type`	bool
`type`	std::integral_constant<bool, value>

[edit]Possible implementation

template<class T>struct is_default_constructible:std::is_constructible<T>{}; template<class T>struct is_trivially_default_constructible:std::is_trivially_constructible<T>{}; template<class T>struct is_nothrow_default_constructible:std::is_nothrow_constructible<T>{};

[edit]Notes

In many implementations,std::is_nothrow_default_constructible also checks if the destructor throws because it is effectivelynoexcept(T()). Same applies tostd::is_trivially_default_constructible, which, in these implementations, also requires that the destructor is trivial:GCC bug 51452,LWG issue 2116.

std::is_default_constructible<T> does not test thatT x; would compile; it attemptsdirect-initialization with an empty argument list (seestd::is_constructible). Thus,std::is_default_constructible_v<constint> andstd::is_default_constructible_v<constint[10]> aretrue.

[edit]Example

Run this code

#include <string>#include <type_traits> struct S1{std::string str;// member has a non-trivial default constructor};static_assert(std::is_default_constructible_v<S1>==true);static_assert(std::is_trivially_default_constructible_v<S1>==false); struct S2{int n;    S2()=default;// trivial and non-throwing};static_assert(std::is_trivially_default_constructible_v<S2>==true);static_assert(std::is_nothrow_default_constructible_v<S2>==true); int main(){}

[edit]See also

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)[edit]
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template)[edit]
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)	checks if a type can be constructed from an rvalue reference (class template)[edit]
default_initializable (C++20)	specifies that an object of a type can be default constructed (concept)[edit]