std::is_nothrow_constructible

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

Defined in header
template< class T, class... Args > struct is_constructible;	(1)	(since C++11)
template< class T, class... Args > struct is_trivially_constructible;	(2)	(since C++11)
template< class T, class... Args > struct is_nothrow_constructible;	(3)	(since C++11)

1%29T是对象或引用类型以及变量定义。T obj(std::declval<Args>()...是格式良好的，提供成员常量。value等于true.在所有其他情况下，value是false...

为了本检查的目的，变量定义永远不会被解释为函数声明，并且使用std::declval不被认为是ODR-使用...存取检查执行时，就像从与以下内容无关的上下文中执行。T中的任何类型Args.只考虑变量定义的直接上下文的有效性。

2%29与1%29相同，但变量定义不调用任何不平凡的操作。为了本检查的目的，调用std::declval被认为是微不足道的。

3%29与1%29相同，但变量定义是noexcept...

T和参数包中的所有类型。Args每一个都是一个完整的类型，%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, class... Args > inline constexpr bool is_constructible_v = is_constructible::value;		(since C++17)
template< class T, class... Args > inline constexpr bool is_trivially_constructible_v = is_trivially_constructible<T, Args...>::value;		(since C++17)
template< class T, class... Args > inline constexpr bool is_nothrow_constructible_v = is_nothrow_constructible<T, Args...>::value;		(since C++17)

继承自STD：积分[医]常量

成员常数

value static	true if T is constructible from Args... , 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>

注记

在许多实现中，is_nothrow_constructible还检查析构函数是否抛出，因为它有效。noexcept(T(arg)).同样适用于is_trivially_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>
 
class Foo {
    int v1;
    double v2;
 public:
    Foo(int n) : v1(n), v2() {}
    Foo(int n, double f) noexcept : v1(n), v2(f) {}
};
 
int main() {
    std::cout << "Foo is ...\n" << std::boolalpha
              << "\tTrivially-constructible from const Foo&? "
              << std::is_trivially_constructible<Foo, const Foo&>::value << '\n'
              << "\tTrivially-constructible from int? "
              << std::is_trivially_constructible<Foo, int>::value << '\n'
              << "\tConstructible from int? "
              << std::is_constructible<Foo, int>::value << '\n'
              << "\tNothrow-constructible from int? "
              << std::is_nothrow_constructible<Foo, int>::value << '\n'
              << "\tNothrow-constructible from int and double? "
              << std::is_nothrow_constructible<Foo, int, double>::value << '\n';
}

二次

产出：

二次

Foo is ...
        Trivially-constructible from const Foo&? true
        Trivially-constructible from int? false
        Constructible from int? true
        Nothrow-constructible from int? false
        Nothrow-constructible from int and double? true

二次

另见

is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)	checks if a type has a default constructor (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)
uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)

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