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std::is_default_constructible

Defined in header <type_traits>
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template< class T >
struct is_default_constructible;
 (1) (since C++11)
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template< class T >
struct is_trivially_default_constructible;
 (2) (since C++11)
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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

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template< class T >
constexpr bool is_default_constructible_v = is_default_constructible<T>::value;
 (since C++17)
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template< class T >
constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;
 (since C++17)
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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

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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

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#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:

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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)
doc_CPP
doc_CPP
2025-01-10 15:47:30
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