std::is_default_constructible
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%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<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) |
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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