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

 

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Language support Type support (basic types, RTTI, type traits)      Library feature-test macros (C++20) Dynamic memory management Program utilities Error handling Coroutine support (C++20) Variadic functions launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20) is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects bitset Formatting library (C++20) hash (C++11) integer_sequence (C++14) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) forward (C++11) move (C++11) move_if_noexcept (C++11) as_const (C++17) to_underlying (C++23) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) Stacktrace stacktrace_entry (C++23) basic_stacktrace (C++23)

 

Type support

Basic types
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ptrdiff_t size_t max_align_t (C++11) byte (C++17)	nullptr_t (C++11) offsetof NULL
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C numeric limits interface
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type_info type_index (C++11)	bad_typeid bad_cast
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Type properties
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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)
Constant evaluation context
is_constant_evaluated (C++20)

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_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)	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) 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_ofinvoke_result (C++11)(until C++20)(C++17) type_identity (C++20)

 

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.

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.

The behavior of a program that adds specializations for any of the templates described on this page 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)