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

 

C++

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Regular expressions library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

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Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Elementary string conversions

to_chars (C++17)

from_chars (C++17)

chars_format (C++17)

Type operations

declval (C++11)

as_const (C++17)

 

Type support

Basic types
Fundamental types
Fixed width integer types (C++11)
ptrdiff_t size_t max_align_t (C++11) byte (C++17)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast
Type traits
Type categories
is_void (C++11) is_null_pointer (C++14) 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_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_reference (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_pod (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11)	is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(deprecated in C++17) is_aggregate (C++17) is_signed (C++11) is_unsigned (C++11)
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_base_of (C++11) is_convertible (C++11)	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) aligned_union (C++11) decay (C++11) enable_if (C++11) void_t (C++17)	conditional (C++11) common_type (C++11) underlying_type (C++11) result_ofinvoke_result (C++11)(deprecated in C++17)(C++17)

 

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 T 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 T 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.

T and all types in the parameter pack Args shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, 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 T 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() {
    std::cout << "Foo is ...\n" << std::boolalpha
              << "\tTrivially-constructible from const Foo&? "
              << std::is_trivially_constructible<Foo, const Foo&>::value << '\n'
              << "\tTrivially-constructible from int? "
              << std::is_trivially_constructible<Foo, int>::value << '\n'
              << "\tConstructible from int? "
              << std::is_constructible<Foo, int>::value << '\n'
              << "\tNothrow-constructible from int? "
              << std::is_nothrow_constructible<Foo, int>::value << '\n'
              << "\tNothrow-constructible from int and double? "
              << std::is_nothrow_constructible<Foo, int, double>::value << '\n';
}

Output:

Foo is ...
        Trivially-constructible from const Foo&? true
        Trivially-constructible from int? false
        Constructible from int? true
        Nothrow-constructible from int? false
        Nothrow-constructible from int and double? true

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)
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)
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)
uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)