std::is_default_constructible

STD：：是[医]违约[医]可构造的，STD：：is[医]琐碎[医]违约[医]可构造的，STD：：is[医]无抛[医]违约[医]可构造

Defined in header
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%29std::is_constructible<T>::value是true，提供成员常量。value等于true，否则value是false...

2%29std::is_trivially_constructible<T>::value是true，提供成员常量。value等于true，否则value是false...

3%29std::is_nothrow_constructible<T>::value是true，提供成员常量。value等于true，否则value是false...

T为完整类型，%28可能是cv-合格%29void，或者一系列未知的界限。否则，行为就没有定义。

辅助变量模板

template< class T > inline constexpr bool is_default_constructible_v = is_default_constructible::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)

继承自STD：积分[医]常量

成员常数

value static	true if T is default-constructible , false otherwise (public static member constant)

成员函数

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (public member function)

成员类型

Type	Definition
value_type	bool
type	std::integral_constant<bool, value>

可能的实施

模板<class T>结构是[医]违约[医]可构造性：std：：is[医]可构造<T>模板<class T>结构是[医]琐碎[医]违约[医]可构造性：std：：is[医]琐碎[医]可构造<T>模板<class T>结构是[医]无抛[医]违约[医]可构造性：std：：is[医]无抛[医]可构造<T>{}；

*。

注记

在许多实现中，is_nothrow_default_constructible还检查析构函数是否抛出，因为它有效。noexcept(T()).同样适用于is_trivially_default_constructible，在这些实现中，还要求析构函数是微不足道的：gcc bug 51452lwg第2116期...

例

二次

#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';
}

二次

产出：

二次

Ex1 is default-constructible? true
Ex1 is trivially default-constructible? false
Ex2 is trivially default-constructible? true
Ex2 is nothrow default-constructible? true

二次

另见

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)	checks if a type can be constructed from an rvalue reference (class template)

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