std:: is_constructible, std:: is_trivially_constructible, std:: is_nothrow_constructible

is_constructible is_trivially_constructible is_nothrow_constructible (C++11) (C++11) (C++11)
is_default_constructible is_trivially_default_constructible is_nothrow_default_constructible (C++11) (C++11) (C++11)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11) (C++11) (C++11)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11) (C++11) (C++11)
is_assignable is_trivially_assignable is_nothrow_assignable (C++11) (C++11) (C++11)

is_copy_assignable is_trivially_copy_assignable is_nothrow_copy_assignable (C++11) (C++11) (C++11)
is_move_assignable is_trivially_move_assignable is_nothrow_move_assignable (C++11) (C++11) (C++11)
is_destructible is_trivially_destructible is_nothrow_destructible (C++11) (C++11) (C++11)
has_virtual_destructor (C++11)
is_swappable_with is_swappable is_nothrow_swappable_with is_nothrow_swappable (C++17) (C++17) (C++17) (C++17)

Relationships and property queries

is_same (C++11)
is_convertible is_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_invocable is_invocable_r is_nothrow_invocable is_nothrow_invocable_r (C++17) (C++17) (C++17) (C++17)

Type modifications

remove_cv remove_const remove_volatile (C++11) (C++11) (C++11)
add_cv add_const add_volatile (C++11) (C++11) (C++11)
make_signed (C++11)
make_unsigned (C++11)

remove_reference (C++11)
add_lvalue_reference add_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)
enable_if (C++11)
void_t (C++17)

conditional (C++11)
common_type (C++11)
common_reference (C++20)
underlying_type (C++11)
result_of invoke_result (C++11) ( until C++20* ) (C++17)
type_identity (C++20)

Compile-time rational arithmetic

Compile-time integer sequences

integer_sequence

(C++14)

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) If

is an object or reference type and the variable definition T obj ( std:: declval < Args > ( ) ... ) ; is well-formed, provides the member constant


        value

equal to true . In all other cases,


        value

is false .
For the purposes of this check, the variable definition is never interpreted as a function declaration, and the use of std::declval is not considered an odr-use . Access checks are performed as if from a context unrelated to

and any of the types in


        Args

. 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 to std::declval is considered trivial.

3) Same as (1) , but the variable definition is


        noexcept

If T or any type in the parameter pack Args 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.

Helper variable templates

template < class T, class ... Args > inline constexpr bool is_constructible_v = is_constructible < T, Args... > :: value ;		(since C++17)
template < class T, class ... Args > inline constexpr bool is_trivially_constructible_v = is_trivially_constructible < T, Args... > :: value ;		(since C++17)
template < class T, class ... Args > inline constexpr bool is_nothrow_constructible_v = is_nothrow_constructible < T, Args... > :: value ;		(since C++17)

Inherited from std:: integral_constant

Member constants

value

[static]

true if

is constructible from


           Args...

, 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

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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

is_default_constructible is_trivially_default_constructible is_nothrow_default_constructible (C++11) (C++11) (C++11)	checks if a type has a default constructor (class template)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11) (C++11) (C++11)	checks if a type has a copy constructor (class template)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11) (C++11) (C++11)	checks if a type can be constructed from an rvalue reference (class template)
constructible_from (C++20)	specifies that a variable of the type can be constructed from or bound to a set of argument types (concept)