Multi-inheritance & NVI

nvi_and_multiple_inheritance.cc

// Demonstrates mild multiple inheritance and Non-Virtual Interface.
// The base classes cannot be pure interfaces; they have private data
// and code, but unambiguous semantics. The only slightly-odd behavior
// is that a static_cast of a derived class to one of the bases shifts
// its memory address (see below).
#include <iostream>
using std::cout; using std::endl;

// A base class.
class Continuous {
 public:
  double GetDerivative() const {
    ++deriv_count_;
    return GetDerivativeImpl();
  }
  int get_num_derivs() const { return deriv_count_; }
  virtual ~Continuous() = default;

 protected:
  Continuous() = default;
  virtual double GetDerivativeImpl() const = 0;

 private:
  mutable int deriv_count_{}; // could be static to get total count
};

// Another base class.
class Discrete {
 public:
  double GetUpdate() const {
    ++update_count_;
    return GetUpdateImpl();
  }
  int get_num_updates() const { return update_count_; }
 virtual ~Discrete() = default;

 protected:
  Discrete() = default;
  virtual double GetUpdateImpl() const = 0;

 private:
  mutable int update_count_{};
};

// Derived class implements protected pure virtuals.
class MySystem : public Continuous, public Discrete {
 private:
  double GetDerivativeImpl() const override { return 1.; }
  double GetUpdateImpl()     const override { return 2.; }
};

// Another derived class.
class YourSystem : public Continuous, public Discrete {
 private:
  double GetDerivativeImpl() const override { return 10.; }
  double GetUpdateImpl()     const override { return 20.; }
};

int main() {
  MySystem sys;
  cout << "Deriv=" << sys.GetDerivative() << endl;
  cout << "Update=" << sys.GetUpdate() << endl;
  cout << "Update=" << sys.GetUpdate() << endl;
  cout << "nderivs=" << sys.get_num_derivs() << endl;
  cout << "nupdates=" << sys.get_num_updates() << endl;
  cout << "&sys=" << &sys << endl;
  cout << "&Cont(sys)=" << &static_cast<Continuous&>(sys) << endl;
  cout << "&Disc(sys)=" << &static_cast<Discrete&>(sys) << endl;

  return 0;
}

Output:

Deriv=1
Update=2
Update=2
nderivs=1
nupdates=2
&sys=0x7ffc8a65e3a0
&Cont(sys)=0x7ffc8a65e3a0
&Disc(sys)=0x7ffc8a65e3b0