std::any::type
来自cppreference.com
const std::type_info& type() const noexcept; |
(C++17 起) | |
查询所含类型。
参数
(无)
返回值
若实例非空则为所含值的 typeid
,否则为 typeid(void) 。
示例
示例演示在编译和运行时注册新观览器的 std::any
观览器手法。
运行此代码
#include <type_traits> #include <any> #include <functional> #include <iomanip> #include <iostream> #include <typeindex> #include <typeinfo> #include <unordered_map> #include <vector> template<class T, class F> inline std::pair<const std::type_index, std::function<void(std::any const&)>> to_any_visitor(F const &f) { return { std::type_index(typeid(T)), [g = f](std::any const &a) { if constexpr (std::is_void_v<T>) g(); else g(std::any_cast<T const&>(a)); } }; } static std::unordered_map< std::type_index, std::function<void(std::any const&)>> any_visitor { to_any_visitor<void>([]{ std::cout << "{}"; }), to_any_visitor<int>([](int x){ std::cout << x; }), to_any_visitor<unsigned>([](unsigned x){ std::cout << x; }), to_any_visitor<float>([](float x){ std::cout << x; }), to_any_visitor<double>([](double x){ std::cout << x; }), to_any_visitor<char const*>([](char const *s) { std::cout << std::quoted(s); }), // ……添加更多你的类型的特化…… }; inline void process(const std::any& a) { if (const auto it = any_visitor.find(std::type_index(a.type())); it != any_visitor.cend()) { it->second(a); } else { std::cout << "Unregistered type "<< std::quoted(a.type().name()); } } template<class T, class F> inline void register_any_visitor(F const& f) { std::cout << "Register visitor for type " << std::quoted(typeid(T).name()) << '\n'; any_visitor.insert(to_any_visitor<T>(f)); } auto main() -> int { std::vector<std::any> va { {}, 42, 123u, 3.14159f, 2.71828, "C++17", }; std::cout << "{ "; for (const std::any& a : va) { process(a); std::cout << ", "; } std::cout << "}\n"; process(std::any(0xFULL)); //< 反注册 "y" 的类型( unsigned long long ) std::cout << '\n'; register_any_visitor<unsigned long long>([](auto x) { std::cout << std::hex << std::showbase << x; }); process(std::any(0xFULL)); //< OK : 0xf std::cout << '\n'; }
可能的输出:
{ {}, 42, 123, 3.14159, 2.71828, "C++17", } Unregistered type "y" Register visitor for type "y" 0xf
参阅
(C++11) |
针对 type_info 对象的包装,它能用作关联容器和无序关联容器的索引。 (类) |