std::is_constructible,std::is_trivially_constructible,std::is_nothrow_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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Date and time library
Input/output library
Filesystem library(C++17)
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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,class...Args> struct is_constructible;	(1)	(since C++11)
template<class T,class...Args> struct is_trivially_constructible;	(2)	(since C++11)
template<class T,class...Args> struct is_nothrow_constructible;	(3)	(since C++11)

1) IfT is an object or reference type and the variable definitionT obj(std::declval<Args>()...); is well-formed, provides the member constantvalue equal totrue. In all other cases,value isfalse.
For the purposes of this check, the variable definition is never interpreted as a function declaration, and the use ofstd::declval is not considered anodr-use.Access checks are performed as if from a context unrelated toT and any of the types inArgs. Only the validity of the immediate context of the variable definition is considered.

2) Same as(1), but the variable definition does not call any operation that is not trivial. For the purposes of this check, the call tostd::declval is considered trivial.

3) Same as(1), but the variable definition isnoexcept.

IfT or any type in the parameter packArgs 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,class...Args> inlineconstexprbool is_constructible_v= is_constructible<T, Args...>::value;		(since C++17)
template<class T,class...Args> inlineconstexprbool is_trivially_constructible_v= is_trivially_constructible<T, Args...>::value;		(since C++17)
template<class T,class...Args> inlineconstexprbool is_nothrow_constructible_v= is_nothrow_constructible<T, Args...>::value;		(since C++17)

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is constructible fromArgs...,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]Notes

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

[edit]Example

Run this code

#include <iostream>#include <type_traits> class Foo{int v1;double v2;public:    Foo(int n): v1(n), v2(){}    Foo(int n,double f)noexcept: v1(n), v2(f){}}; int main(){auto is=[](bool o){return(o?"\t""is ":"\t""isn't ");};std::cout<<"Foo ...\n"<< is(std::is_trivially_constructible_v<Foo,const Foo&>)<<"Trivially-constructible from const Foo&\n"<< is(std::is_trivially_constructible_v<Foo,int>)<<"Trivially-constructible from int\n"<< is(std::is_constructible_v<Foo,int>)<<"Constructible from int\n"<< is(std::is_nothrow_constructible_v<Foo,int>)<<"Nothrow-constructible from int\n"<< is(std::is_nothrow_constructible_v<Foo,int,double>)<<"Nothrow-constructible from int and double\n";}

Output:

Foo ...        is Trivially-constructible from const Foo&        isn't Trivially-constructible from int        is Constructible from int        isn't Nothrow-constructible from int        is Nothrow-constructible from int and double

[edit]See also

is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)	checks if a type has a default constructor (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]
constructible_from (C++20)	specifies that a variable of the type can be constructed from or bound to a set of argument types (concept)[edit]