Hand made polymorphism in C++11

class_demo.cc

#include <cstdio>

// ==========================================

template<class T>
struct VTableFor;

// ==========================================

struct Vehicle{
	void *ptr;
	const VTableFor<Vehicle> *vptr;

	template<class T>
	Vehicle(T *ptr) : ptr(ptr), vptr(& T::vtable__){}

	template<class T>
	Vehicle(T &ptr) : Vehicle(&ptr){}

	void accelerate();
};

template<>
struct VTableFor<Vehicle>{
	void (*accelerate)(void *ptr);
};

inline void Vehicle::accelerate(){
	vptr->accelerate(ptr);
}

// ==========================================

struct Car{
	Car(const char *name) : name(name){}

	const char *name;

	static const VTableFor<Vehicle> vtable__;
};

const VTableFor<Vehicle> Car::vtable__{
	[](void *ptr){
		printf("Car::accelerate, Name : %s\n", reinterpret_cast<Car *>(ptr)->name);
	}
};

// ==========================================

struct Truck{
	Truck(int const tonnage) : tonnage(tonnage){}

	int tonnage;

	static const VTableFor<Vehicle> vtable__;
};

const VTableFor<Vehicle> Truck::vtable__{
	[](void *ptr){
		printf("Truck::accelerate, Tonnage: %d\n", reinterpret_cast<Truck *>(ptr)->tonnage);
	}
};

// ==========================================

#include <vector>

void acc(Vehicle x){
	x.accelerate();
}

int main(){
	Car   bmw("BMW");
	Truck t100(100);

	acc(bmw);
	acc(t100);
}

class_demo_constexpr_cpp11.cc

#include <cstdio>

// ==========================================

template<class T>
struct VTableFor;

// ==========================================

struct Vehicle{
	void *ptr;
	const VTableFor<Vehicle> *vptr;

	template<class T>
	constexpr Vehicle(T *ptr) : ptr(ptr), vptr(& T::vtable__){}

	template<class T>
	constexpr Vehicle(T &ptr) : Vehicle(&ptr){}

	void accelerate();
};

template<>
struct VTableFor<Vehicle>{
	void (*accelerate)(void *ptr);
};

inline void Vehicle::accelerate(){
	vptr->accelerate(ptr);
}

// ==========================================

struct Car{
	constexpr Car(const char *name) : name(name){}

	const char *name;

	static void accelerate__(void *ptr){
		printf("Car::accelerate, Name : %s\n", reinterpret_cast<Car *>(ptr)->name);
	}

	static constexpr VTableFor<Vehicle> vtable__ = {
		Car::accelerate__
	};
};

constexpr VTableFor<Vehicle> Car::vtable__;

// ==========================================

struct Truck{
	constexpr Truck(int const tonnage) : tonnage(tonnage){}

	int tonnage;

	static void accelerate__(void *ptr){
		printf("Truck::accelerate, Tonnage: %d\n", reinterpret_cast<Truck *>(ptr)->tonnage);
	}

	static constexpr VTableFor<Vehicle> vtable__{
		Truck::accelerate__
	};
};

constexpr VTableFor<Vehicle> Truck::vtable__;

// ==========================================

#include <vector>

void acc(Vehicle x){
	x.accelerate();
}

int main(){
	Car   bmw("BMW");
	Truck t100(100);

	acc(bmw);
	acc(t100);
}

normal_demo.cc

#include <cstdio>

// ==========================================

struct Vehicle{
	virtual void accelerate() = 0;
};

// ==========================================

struct Car : Vehicle{
	Car(const char *name) : name(name){}

	const char *name;

	void accelerate() override{
		printf("V Car::accelerate, Name : %s\n", this->name);
	}
};

// ==========================================

struct Truck : Vehicle{
	Truck(int const tonnage) : tonnage(tonnage){}

	int tonnage;

	void accelerate() override{
		printf("V Truck::accelerate, Tonnage: %d\n", this->tonnage);
	}
};

// ==========================================

#include <vector>

void acc(Vehicle &x){
	x.accelerate();
}

int main(){
	Car   bmw("BMW");
	Truck t100(100);

	acc(bmw);
	acc(t100);
}