std::is_default_constructible, std::is_trivially_default_constructible, std::is_nothrow_default_constructible
| 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.
| Contents | 
[edit] Helper variable templates
| 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) | |
Inherited from std::integral_constant
Member constants
| value [static] | true if Tis 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> | 
[edit] 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> {}; | 
[edit] 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. 
[edit] 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
[edit] 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 can be constructed from an rvalue reference (class template) | 


