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

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is_default_constructibleis_trivially_default_constructibleis_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) Provides the member constant value equal to std::is_constructible<T>::value.
2) Provides the member constant value equal to std::is_trivially_constructible<T>::value.
3) Provides the member constant value equal to std::is_nothrow_constructible<T>::value.

T shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

Contents

[edit] Helper variable templates

template< class T >

inline constexpr bool is_default_constructible_v =

    is_default_constructible<T>::value;
(since C++17)
template< class T >

inline constexpr bool is_trivially_default_constructible_v =

    is_trivially_default_constructible<T>::value;
(since C++17)
template< class T >

inline 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, std::is_nothrow_default_constructible also checks if the destructor throws because it is effectively noexcept(T()). Same applies to std::is_trivially_default_constructible, which, in these implementations, also requires that the destructor is trivial: GCC bug 51452, LWG issue 2116.

std::is_default_constructible<T> does not test that T x; would compile; it attempts direct-initialization with an empty argument list (see std::is_constructible). Thus, std::is_default_constructible_v<const int> and std::is_default_constructible_v<const int[10]> are true.

[edit] Example

#include <string>
#include <type_traits>
 
struct S1
{
    std::string str; // member has a non-trivial default constructor
};
static_assert(std::is_default_constructible_v<S1> == true);
static_assert(std::is_trivially_default_constructible_v<S1> == false);
 
struct S2
{
    int n;
    S2() = default; // trivial and non-throwing
};
static_assert(std::is_trivially_default_constructible_v<S2> == true);
static_assert(std::is_nothrow_default_constructible_v<S2> == true);
 
int main() {}

[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]
specifies that an object of a type can be default constructed
(concept) [edit]