Defined in header <type_traits> | ||
---|---|---|
template< class T > struct is_default_constructible; | (1) | (since C++11) |
template< class T > struct is_trivially_default_constructible; | (2) | (since C++11) |
template< class T > struct is_nothrow_default_constructible; | (3) | (since C++11) |
1) If std::is_constructible<T>::value
is true
, provides the member constant value
equal to true
, otherwise value
is false
.
2) If std::is_trivially_constructible<T>::value
is true
, provides the member constant value
equal to true
, otherwise value
is false
.
3) If std::is_nothrow_constructible<T>::value
is true
, provides the member constant value
equal to true
, otherwise value
is false
.
T
shall 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.
template< class T > inline constexpr bool is_default_constructible_v = is_default_constructible<T>::value; | (since C++17) | |
template< class T > inline constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value; | (since C++17) | |
template< class T > inline constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value; | (since C++17) |
value
[static] | true if T is default-constructible , false otherwise (public static member constant) |
operator bool | converts the object to bool , returns value (public member function) |
operator()
(C++14) | returns value (public member function) |
Type | Definition |
---|---|
value_type | bool |
type | std::integral_constant<bool, value> |
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> {}; |
In many implementations, is_nothrow_default_constructible
also checks if the destructor throws because it is effectively noexcept(T())
. Same applies to 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
.
#include <iostream> #include <type_traits> struct Ex1 { std::string str; // member has a non-trivial default ctor }; struct Ex2 { int n; Ex2() = default; // trivial and non-throwing }; int main() { std::cout << std::boolalpha << "Ex1 is default-constructible? " << std::is_default_constructible<Ex1>::value << '\n' << "Ex1 is trivially default-constructible? " << std::is_trivially_default_constructible<Ex1>::value << '\n' << "Ex2 is trivially default-constructible? " << std::is_trivially_default_constructible<Ex2>::value << '\n' << "Ex2 is nothrow default-constructible? " << std::is_nothrow_default_constructible<Ex2>::value << '\n'; }
Output:
Ex1 is default-constructible? true Ex1 is trivially default-constructible? false Ex2 is trivially default-constructible? true Ex2 is nothrow default-constructible? true
(C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template) |
(C++11)(C++11)(C++11) | checks if a type has a copy constructor (class template) |
(C++11)(C++11)(C++11) | checks if a type can be constructed from an rvalue reference (class template) |
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