std::void_t
来自 cppreference.cn
| 定义于头文件 <type_traits> |
||
| template< class... > using void_t = void; |
(自 C++17 起) | |
实用元函数，将任何类型的序列映射到类型 void。此元函数是在 C++20 的 概念 之前利用 SFINAE 的便捷方式，尤其用于根据表达式在未求值语境（例如 decltype 表达式的操作数）中是否有效来有条件地从候选集中移除函数，从而允许基于支持的操作存在单独的函数重载或特化。
[编辑] 注释
此元函数用于模板元编程，以检测 SFINAE 上下文中的病构类型
// primary template handles types that have no nested ::type member: template<class, class = void> struct has_type_member : std::false_type {}; // specialization recognizes types that do have a nested ::type member: template<class T> struct has_type_member<T, std::void_t<typename T::type>> : std::true_type {};
它也可用于检测表达式的有效性
// primary template handles types that do not support pre-increment: template<class, class = void> struct has_pre_increment_member : std::false_type {}; // specialization recognizes types that do support pre-increment: template<class T> struct has_pre_increment_member<T, std::void_t<decltype( ++std::declval<T&>() )> > : std::true_type {};
在 CWG issue 1558（C++11 缺陷）解决之前，别名模板 中未使用的参数不能保证确保 SFINAE 并且可能被忽略，因此早期的编译器需要更复杂的 void_t 定义，例如
template<typename... Ts> struct make_void { typedef void type; }; template<typename... Ts> using void_t = typename make_void<Ts...>::type;
| 特性测试 宏 | 值 | Std | 特性 |
|---|---|---|---|
__cpp_lib_void_t |
201411L |
(C++17) | std::void_t
|
[编辑] 示例
运行此代码
#include <iomanip> #include <iostream> #include <map> #include <type_traits> #include <vector> // Variable template that checks if a type has begin() and end() member functions template<typename, typename = void> constexpr bool is_iterable = false; template<typename T> constexpr bool is_iterable< T, std::void_t<decltype(std::declval<T>().begin()), decltype(std::declval<T>().end()) > > = true; // An iterator trait those value_type is the value_type of the iterated container, // supports even back_insert_iterator (where value_type is void) template<typename T, typename = void> struct iterator_trait : std::iterator_traits<T> {}; template<typename T> struct iterator_trait<T, std::void_t<typename T::container_type>> : std::iterator_traits<typename T::container_type::iterator> {}; class A {}; #define SHOW(...) std::cout << std::setw(34) << #__VA_ARGS__ \ << " == " << __VA_ARGS__ << '\n' int main() { std::cout << std::boolalpha << std::left; SHOW(is_iterable<std::vector<double>>); SHOW(is_iterable<std::map<int, double>>); SHOW(is_iterable<double>); SHOW(is_iterable<A>); using container_t = std::vector<int>; container_t v; static_assert(std::is_same_v< container_t::value_type, iterator_trait<decltype(std::begin(v))>::value_type >); static_assert(std::is_same_v< container_t::value_type, iterator_trait<decltype(std::back_inserter(v))>::value_type >); }
输出
is_iterable<std::vector<double>> == true is_iterable<std::map<int, double>> == true is_iterable<double> == false is_iterable<A> == false
[编辑] 参见
| (C++11) |
有条件地从重载决议中移除函数重载或模板特化 (类模板) |
