Virtual in C++

polymorphism.c

///////////////////////////////////////////////////////////////////////////////////////////////
// Demonstrates a sort of pseudo-polymorphism in C using similar methods to the C++ compiler //
///////////////////////////////////////////////////////////////////////////////////////////////

typedef struct vtable { // Virtual table for holding methods
    int (*process)(int); // Pointer to a function that returns an int and takes in an int
} vtable;

typedef struct NumberProcessor { // "class" that processes an integer
    const vtable* vtable; // Method table
} NumberProcessor;

int square(int a) { return a*a; }
int third(int a) { return a*a*a; }

const vtable vsquare = { square };
const vtable vthird = { third };

int proc(NumberProcessor* nproc, int n) { // "polymorphic" function
    return nproc->vtable->process(n); // Get function ptr from vtable
}

int main() {

    NumberProcessor square_ptr = { &vsquare }; // Squares a number
    NumberProcessor third_ptr = { &vthird }; // Computes n*n*n
    
    printf("%i\n", proc(&square_ptr, 5)); // Prints 25 (5*5)
    printf("%i\n", proc(&third_ptr, 3)); // Prints 27 (3*3*3)

}

virtual.cpp

//////////////////////////////////////////////////////////
// Demonstrates the usage of the virtual keyword in C++ //
//////////////////////////////////////////////////////////

#include <iostream>

// Structs work the same as classes in C++ except their members are public by default
struct base {
    virtual int get() { return 5; }
};

struct derived : public base {
    int get() { return 7; }
};

int get(base* b) { // Polymorphic function
    return b->get();
}

int main() {
  
    base* b1 = new Base;
  
    std::cout << b1->get() << std::endl; // Prints 5
    
    std::cout << get(b1) << std::endl; // Prints 5

    base* b2 = new derived;

    std::cout << b2->get() << std::endl; // Prints 7
    
    std::cout << get(b2) << std::endl; // Prints 7

    derived* d = new derived;

    std::cout << d->get() << std::endl; // Prints 7
  
    // Cleanup
    delete b1;
    delete b2;
    delete d;

}