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) |
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.
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) |
| value
[static] | true if T is default-constructible, false otherwise (public static member constant) |
| operator bool | converts the object to bool, returns value (public member function) |
| operator()
(C++14) | returns value (public member function) |
| Type | Definition |
|---|---|
value_type | bool |
type | std::integral_constant<bool, value> |
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> {}; |
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.
#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
|
(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 can be constructed from an rvalue reference (class template) |
|
(C++20) | specifies that an object of a type can be default constructed (concept) |
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