Fresh as hell demonstration of perfect forwarding constructor with varargs in C++

widget.cpp

#include <iostream>

#include <boost/make_shared.hpp>

class Widget {
 public:
  virtual int getMass() const = 0;
};

class SimpleWidget : Widget {
 private:
  int mass;
  float price;

 public:
  SimpleWidget(int mass, float price) : mass(mass), price(price) {}
  virtual int getMass() const {
    std::cerr << "calculating mass ..." << '\n';
    return mass;
  }
};

template <typename UnderlyingWidget>
class CachingWidget : Widget {
 private:
  UnderlyingWidget underlying;
  mutable int cachedMass;

 public:
  template <typename... Args>
  CachingWidget(Args&&... args) : underlying(std::forward<Args>(args)...) {}

  virtual int getMass() const {
    if (cachedMass == 0) cachedMass = underlying.getMass();
    return cachedMass;
  }
};

int main(int argc, char** argv) {
  boost::shared_ptr<CachingWidget<SimpleWidget>> cachedWidget =
      boost::make_shared<CachingWidget<SimpleWidget>>(31337, 69.99);

  int mass;
  mass = cachedWidget->getMass();
  std::cerr << "mass = " << mass << '\n';
  mass = cachedWidget->getMass();
  std::cerr << "mass = " << mass << '\n';
}