1 #include <iostream> 2 #include <string> 3 template<class T> struct S { 4 S(T arg) { 5 std::cout << typeid(T).name() << std::endl; 6 } 7 }; 8 9 int main() 10 { 11 S<const char*> s{"hello"}; // deduced to S<std::string> 12 }
类模板的使用，需要指定模板参数。自从C++17起，支持根据构造函数的实际参数，推导类模板的类型参数。
#include <iostream> #include <string> template<class T> struct S { S(T arg) { std::cout << typeid(T).name() << std::endl; } }; int main() { S s{"hello"}; // deduced to S<std::string> }
用户还能干预推导，通过指定一个User-defined deduction guides
1 #include <iostream> 2 #include <string> 3 template<class T> struct S { 4 S(T arg) { 5 std::cout << typeid(T).name() << std::endl; 6 } 7 }; 8 S(char const*) -> S<std::string>; 9 int main() 10 { 11 S s{"hello"}; // deduced to S<std::string> 12 }
第8行，指示编译器，当遇到char const*参数时，就把T推导成std::string
参考：http://en.cppreference.com/w/cpp/language/class_template_argument_deduction
https://stackoverflow.com/questions/40951697/what-are-template-deduction-guides-and-when-should-we-use-them
