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

 

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is_default_constructible is_trivially_default_constructible is_nothrow_default_constructible (C++11) (C++11) (C++11)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11) (C++11) (C++11)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11) (C++11) (C++11)
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is_copy_assignable is_trivially_copy_assignable is_nothrow_copy_assignable (C++11) (C++11) (C++11)
is_move_assignable is_trivially_move_assignable is_nothrow_move_assignable (C++11) (C++11) (C++11)
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integral_constant (C++11)

 

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 DefaultConstructible, 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)

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> {};

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

is_constructible is_trivially_constructible is_nothrow_constructible (C++11) (C++11) (C++11)	checks if a type has a constructor for specific arguments (class template)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11) (C++11) (C++11)	checks if a type has a copy constructor (class template)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11) (C++11) (C++11)	checks if a type has a move constructor (class template)