std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible (3) Linux Manual Page
std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible – std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_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) 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.
Helper variable templates
template< class T > (since C++17)
inline constexpr bool is_default_constructible_v = is_default_constructible<T>::value;
template< class T > (since C++17)
inline constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;
template< class T > (since C++17)
inline constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value;
Inherited from std::integral_constant
Member constants
value true if T is default-constructible , false otherwise
[static]
Member functions
operator bool (public member function)
operator() returns value
(C++14)
Member types
Type Definition
value_type bool
type std::integral_constant<bool, value>
Possible implementation
Notes
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.
Example
// Run this code
#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:
See also
is_constructible
is_trivially_constructible
is_nothrow_constructible checks if a type has a constructor for specific arguments
(C++11)
(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)
(C++11)
(C++11)
is_move_constructible
is_trivially_move_constructible
is_nothrow_move_constructible checks if a type can be constructed from an rvalue reference
(C++11)
(C++11)
(C++11)