🔴 手写三份和模板一份，结果一模一样

（本条路线统一用 g++ -std=c++17 -O0 -Wall 编译）上半段各写一份，下半段写一份：

#include <iostream>
#include <string>

/* 不用模板：同一件事对每种类型各写一遍 */
int    twice_int(int x)                 { return x + x; }
double twice_double(double x)           { return x + x; }
std::string twice_str(std::string x)    { return x + x; }

/* 用模板：写一遍，三种类型都能用 */
template <typename T>
T twice(T x) { return x + x; }

int main() {
    std::cout << twice_int(5) << "/" << twice_double(1.5) << "/"
              << twice_str("ab") << "/"
              << twice(5) << "/" << twice(1.5) << "/"
              << twice(std::string("ab")) << "\n";
    return 0;
}