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std::is_default_constructible,std::is_trivially_default_constructible,(3) | C++ Standard Libary | std::is_default_constructible,std::is_trivially_default_constructible,(3) |
NAME¶
std::is_default_constructible,std::is_trivially_default_constructible, - std::is_default_constructible,std::is_trivially_default_constructible,
Synopsis¶
Defined in header <type_traits>
template< class T > (1) (since C++11)
struct is_default_constructible;
template< class T > (2) (since C++11)
struct is_trivially_default_constructible;
template< class T > (3) (since C++11)
struct is_nothrow_default_constructible;
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.
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.
Helper variable templates
template< class T >
inline constexpr bool is_default_constructible_v = (since C++17)
is_default_constructible<T>::value;
template< class T >
inline constexpr bool is_trivially_default_constructible_v = (since
C++17)
is_trivially_default_constructible<T>::value;
template< class T >
inline constexpr bool is_nothrow_default_constructible_v = (since
C++17)
is_nothrow_default_constructible<T>::value;
Inherited from std::integral_constant
Member constants¶
value true if T is default-constructible , false otherwise
[static] (public static member constant)
Member functions¶
operator bool converts the object to bool, returns value
(public member function)
operator() returns value
(C++14) (public member function)
Member types¶
Type Definition
value_type bool
type std::integral_constant<bool, value>
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> {};
Notes¶
In many implementations, is_nothrow_default_constructible also
checks if the
destructor throws because it is effectively noexcept(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.
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.
Example¶
// Run this code
#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
See also¶
is_constructible
is_trivially_constructible checks if a type has a constructor for specific
is_nothrow_constructible arguments
(C++11) (class template)
(C++11)
(C++11)
is_copy_constructible
is_trivially_copy_constructible
is_nothrow_copy_constructible checks if a type has a copy constructor
(C++11) (class template)
(C++11)
(C++11)
is_move_constructible
is_trivially_move_constructible checks if a type can be constructed from an
rvalue
is_nothrow_move_constructible reference
(C++11) (class template)
(C++11)
(C++11)
default_initializable specifies that an object of a type can be default
(C++20) constructed
(concept)
2022.07.31 | http://cppreference.com |