std::is_default_constructible, std::is_trivially_default_constructible, std::is_nothrow_default_constructible

 

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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) Checks whether a type is Template:concept, i.e. has an accessible explicit or implicit default constructor. If the requirement is met, a member constant value equal true is provided, otherwise value is false.

2) Same as 1), but the constructor expression does not call any operation that is not trivial.

3) Same as 1), but the constructor expression is noexcept.

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_default_constructible also checks if the destructor throws because it is effectively noexept(T()): GCC bug 51452 LWG issue 2116.

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:

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

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)	checks if a type can be constructed from an rvalue reference (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)