NAME¶

std::is_default_constructible,std::is_trivially_default_constructible, - std::is_default_constructible,std::is_trivially_default_constructible,

Synopsis¶

Defined in header <type_traits>
template< class T > (1) (since C++11)
struct is_default_constructible;
template< class T > (2) (since C++11)
struct is_trivially_default_constructible;
template< class T > (3) (since C++11)
struct is_nothrow_default_constructible;

1) Provides the member constant value equal to std::is_constructible<T>::value.
2) Provides the member constant value equal to
std::is_trivially_constructible<T>::value.
3) Provides the member constant value equal to
std::is_nothrow_constructible<T>::value.

If T 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.

Member constants¶

value true if T is default-constructible, 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>

Possible implementation¶

template<class T>
struct is_default_constructible : std::is_constructible<T> {};

template<class T>
struct is_trivially_default_constructible : std::is_trivially_constructible<T> {};

template<class T>
struct is_nothrow_default_constructible : std::is_nothrow_constructible<T> {};

Notes¶

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

std::is_default_constructible<T> does not test that T x; would compile; it attempts
direct-initialization with an empty argument list (see std::is_constructible). Thus,
std::is_default_constructible_v<const int> and std::is_default_constructible_v<const
int[10]> are true.

Example¶

// Run this code

#include <string>
#include <type_traits>

struct S1
{
std::string str; // member has a non-trivial default constructor
};
static_assert(std::is_default_constructible_v<S1> == true);
static_assert(std::is_trivially_default_constructible_v<S1> == false);

struct S2
{
int n;
S2() = default; // trivial and non-throwing
};
static_assert(std::is_trivially_default_constructible_v<S2> == true);
static_assert(std::is_nothrow_default_constructible_v<S2> == true);

int main() {}

See also¶

is_constructible
is_trivially_constructible checks if a type has a constructor for specific
is_nothrow_constructible arguments
(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 rvalue
is_nothrow_move_constructible reference
(C++11) (class template)
(C++11)
(C++11)
default_initializable specifies that an object of a type can be default
(C++20) constructed
(concept)