text: #include <dir/file.h>
  
  #include <iostream>
  
  class Implementation : dir::Interface {
  public:
      Implementation() = default;
      ~Implementation() override = default;
      std::string function(int x) const override {
          return std::to_string(x);
      }
  };
  
  int main() {
      const Implementation impl;
  
      std::cout << dir::square(12) << std::endl;
      std::cout << impl.function(12) << std::endl;
  }
  