request_handler.h

#ifndef REQUEST_HANDLER_H
#define REQUEST_HANDLER_H

#include <map>
#include <memory>
#include <string>

class RequestHandler {
 public:
  static RequestHandler* CreateByName(const char* type);
  virtual void HandleRequest(void) = 0;
};

// Notes:
// * The trick here is that you can declare an object at file scope, but you
//   can't do anything else, such as set a map key. But you can get around this
//   by creating a class that does work in its constructor.
// * request_handler_builders must be a pointer. Otherwise, it won't necessarily
//   exist when the RequestHandlerRegisterer constructor gets called.

extern std::map<std::string, RequestHandler* (*)(void)>* request_handler_builders;
template<typename T>
class RequestHandlerRegisterer {
 public:
  RequestHandlerRegisterer(const std::string& type) {
    if (request_handler_builders == nullptr) {
      request_handler_builders = new std::map<std::string, RequestHandler* (*)(void)>;
    }
    (*request_handler_builders)[type] = RequestHandlerRegisterer::Create;
  }
  static RequestHandler* Create() {
    return new T;
  }
};
#define REGISTER_REQUEST_HANDLER(ClassName) \
  static RequestHandlerRegisterer<ClassName> ClassName##__registerer(#ClassName)

#endif  // REQUEST_HANDLER_H