template_adapter_pattern.cc

#include <math.h>
#include <iostream>
#include <vector>

template <class Part>
struct PartScorer {
  struct PartAdapter {
    double PartArea(const Part& part) = delete;
  };

  double ScoreParts(std::vector<Part> parts) {
    PartAdapter part_adapter;
    double total = 0.0;
    for (const Part& part : parts) {
      total += part_adapter.PartArea(part);
    }
    return total;
  }
};

// pretend these are defined elswhere and I don't want to modify them

struct SteeringWheel {
  int radius;
};

struct Seat {
  int height;
  int width;
};

// I just need to define adpaters for any types I want to score

template <>
struct PartScorer<SteeringWheel>::PartAdapter {
  double PartArea(const SteeringWheel& steering_wheel) { return M_PI * steering_wheel.radius * steering_wheel.radius; }
};

template <>
struct PartScorer<Seat>::PartAdapter {
  double PartArea(const Seat& seat) { return seat.height * seat.width; }
};

// And now I can instantiate a PartScorer for these types

int main() {
  std::vector<SteeringWheel> sws = {{22}, {33}, {44}};
  std::vector<Seat> seats = {{10, 20}, {22, 33}, {1, 33}};
  std::cout << "score steering wheels " << PartScorer<SteeringWheel>().ScoreParts(sws) << std::endl;
  std::cout << "score seats " << PartScorer<Seat>().ScoreParts(seats) << std::endl;
}