| 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.
Helper variable templates
template< class T > constexpr bool is_default_constructible_v = is_default_constructible<T>::value; | (since C++17) | |
template< class T > constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value; | (since C++17) | |
template< class T > constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value; | (since C++17) |
Inherited from std::integral_constant
Member constants
| value [static] | true if T is default-constructible , 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> |
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, is_nothrow_default_constructible also checks if the destructor throws because it is effectively noexept(T()): GCC bug 51452 LWG issue 2116.
Example
#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
See also
| (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 has a move constructor (class template) |
| std::experimental::is_default_constructible_v
(library fundamentals TS) | variable template alias of std::is_default_constructible::value (variable template) |
| std::experimental::is_trivially_default_constructible_v
(library fundamentals TS) | variable template alias of std::is_trivially_default_constructible::value (variable template) |
| std::experimental::is_nothrow_default_constructible_v
(library fundamentals TS) | variable template alias of std::is_nothrow_default_constructible::value (variable template) |
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