Editing Virtual function (section)

=== C++ ===
[[Image:ClassDiagram for VirtualFunction.png|400px|thumb|right|Class Diagram of Animal]]
For example, a base class <code>Animal</code> could have a virtual function <code>Eat</code>. Subclass <code>Llama</code> would implement <code>Eat</code> differently than subclass <code>Wolf</code>, but one can invoke <code>Eat</code> on any class instance referred to as Animal, and get the <code>Eat</code> behavior of the specific subclass.

<syntaxhighlight lang="cpp">
class Animal {
 public:
  // Intentionally not virtual:
  void Move() {
    std::cout << "This animal moves in some way" << std::endl;
  }
  virtual void Eat() = 0;
};

// The class "Animal" may possess a definition for Eat if desired.
class Llama : public Animal {
 public:
  // The non virtual function Move is inherited but not overridden.
  void Eat() override {
    std::cout << "Llamas eat grass!" << std::endl;
  }
};
</syntaxhighlight>

This allows a programmer to process a list of objects of class <code>Animal</code>, telling each in turn to eat (by calling <code>Eat</code>), without needing to know what kind of animal may be in the list, how each animal eats, or what the complete set of possible animal types might be.

In C, the mechanism behind virtual functions could be provided in the following manner:
<syntaxhighlight lang="c">
#include <stdio.h>

/* an object points to its class... */
struct Animal {
    const struct AnimalVTable *vtable;
};

/* which contains the virtual function Animal.Eat */
struct AnimalVTable {
    void (*Eat)(struct Animal *self); // 'virtual' function 
};

/*
   Since Animal.Move is not a virtual function
   it is not in the structure above.
*/
void Move(const struct Animal *self) {
    printf("<Animal at %p> moved in some way\n", ( void* )(self));
}

/*
   unlike Move, which executes Animal.Move directly,
   Eat cannot know which function (if any) to call at compile time.
   Animal.Eat can only be resolved at run time when Eat is called.
*/
void Eat(struct Animal *self) {
    const struct AnimalVTable *vtable = self->vtable;
    if (vtable->Eat != NULL) { 
        (*vtable->Eat)(self); // execute Animal.Eat
    } else {
        fprintf(stderr, "'Eat' virtual method not implemented\n");
    }
}

/*
   implementation of Llama.Eat this is the target function 
   to be called by 'void Eat(struct Animal *self).'
*/
static void _Llama_eat(struct Animal *self) {
    printf("<Llama at %p> Llamas eat grass!\n", ( void* )(self));    
}

/* initialize class */
const struct AnimalVTable Animal = { NULL }; // base class does not implement Animal.Eat
const struct AnimalVTable Llama = { _Llama_eat };  // but the derived class does

int main(void) {
   /* init objects as instance of its class */
   struct Animal animal = { &Animal };
   struct Animal llama = { &Llama };
   Move(&animal); // Animal.Move
   Move(&llama);  // Llama.Move
   Eat(&animal);  // cannot resolve Animal.Eat so print "Not Implemented" to stderr
   Eat(&llama);   // resolves Llama.Eat and executes
}
</syntaxhighlight>