322 字
2 分钟
15SFINAE
SFINAE(Substitution Failure Is Not An Error)
工作原理：在模板实例化过程中，编译器会尝试将模板参数替换为具体的类型。 如果在替换过程中出错，编译器不会报错，而是将该模板是为不可行， 继续尝试其他模板或重载，直到找到有效的匹配。 这一特性允许开发者根据类型特性选择不同的模板实现。
std::enable_if (C++11)
concept (C++20)
template<typename T>class has_foo{private: typedef char yes[1]; typedef char no[2]; template<typename U, void(U::*)()> SFINAE{};
template<typename U> static yes& test(SFINAE<U, &U::foo>*);
template<typename U> static no& test(...)public: static constexpr bool value = sizeof(test<T>(0)) == sizeof(yes);};类型萃取 (C++17)
通过类型萃取，来判断一个数据是否为container(即：内部包含有类型的数据。如：vector<int>，内部包含int类型)
class: false_type && true_type
// defualt, extend from false_type// std::false_type has a member called "value"; default, value = false;// has_value_type::value = false;template<typenam T, typename = void>class has_value_type: std::false_type{};
// extend from true_type// std::true_type has a member called "value"; default, value = true;template<typename T>class has_value_type<T, std::enable_if<!std::is_void_v<typename T::value_type>>>::std::true_type{};
// general template
// if T is not void, has_value_type<T> extends from std::true_type.// then, has_value_type<T> has a member called "value" which is true;template<typename T, bool HasValueType = has_value_type<T>::value>class TypePrinter;
// specific templatetemplate<typename T, true>class TypePrinter{public: static void print(){ std::cout << "T has a value_type member" << std::endl; }};
// specific templatetemplate<typename T, false>class TypePrinter{public: static void print(){ std::cout << "T does not have a value_type member" << std::endl; }};