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

From Cppreference

Defined in header `<type_traits>`

template< class T > struct is_default_constructible;	(1)	(C++11 feature)

template< class T > struct is_trivially_default_constructible;	(2)	(C++11 feature)

template< class T > struct is_nothrow_default_constructible;	(3)	(C++11 feature)

1) If the expression T(); is well-formed (that is, T is a scalar or a class with an accessible default constructor), provides the member constant value equal true. For any other type, 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`	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>

[edit] Equivalent definition

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] 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

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)