NAME¶

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

Synopsis¶

Defined in header <type_traits>
template< class T, class... Args > (1) (since C++11)
struct is_constructible;
template< class T, class... Args > (2) (since C++11)
struct is_trivially_constructible;
template< class T, class... Args > (3) (since C++11)
struct is_nothrow_constructible;

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

Inherited from std::integral_constant

Member constants¶

value true if T is constructible from Args... , false otherwise
[static] (public static member constant)

Member functions¶

operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (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

See also¶

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