Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | types
 
 
 
Type support
Type properties
(C++11)
(C++11)
(C++11)
(C++14)
Type trait constants
Metafunctions
(C++17)
Supported operations
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible
(C++11)(C++11)(C++11)
Relationships and property queries
(C++11)
(C++11)
(C++11)
(C++11)
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)
(C++11)
(C++17)
(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) 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 >
constexpr bool is_default_constructible_v = is_default_constructible<T>::value;
(since C++17)
template< class T >
constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;
(since C++17)
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>

[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 noexept(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

checks if a type has a constructor for specific arguments
(class template) [edit]
checks if a type has a copy constructor
(class template) [edit]
checks if a type can be constructed from an rvalue reference
(class template) [edit]
variable template alias of std::is_default_constructible::value
(variable template) [edit]
variable template alias of std::is_trivially_default_constructible::value
(variable template) [edit]
variable template alias of std::is_nothrow_default_constructible::value
(variable template) [edit]