Virtual function: Difference between revisions

Revision as of 22:16, 8 March 2024

Revision as of 23:38, 16 May 2024

Line 52:

/* an object points to its class... */

struct Animal {

const struct AnimalClassAnimalVTable * classvtable;

};

/* which contains the virtual function Animal.Eat */

struct AnimalClassAnimalVTable {

void (*Eat)(struct Animal *self); // 'virtual' function

};

/*

/* Since Animal.Move is not a virtual function

itSince Animal.Move is not ina the structure above.virtual */function

it is not in the structure above.

void Move(struct Animal * self)

*/

{

void Move(const struct Animal * self) {

printf("<Animal at %p> moved in some way\n", (void 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 AnimalClassAnimalVTable * classvtable = *( const void **) )(self);

{

if (vtable->Eat != NULL) {

const struct AnimalClass * class = *(const void **) self;

if (class*vtable->Eat)(self); // execute Animal.Eat

} else {

class->Eat(self); // execute Animal.Eat

fprintf(stderr, "'Eat' virtual method not implemented\n");

else

}

fprintf(stderr, "Eat not implemented\n");

}

/*

/* implementation of Llama.Eat this is the target function

to be called by 'void Eat(struct Animal *).' */

static to be called by 'void _Llama_eatEat(struct Animal * self).'

*/

{

static void _Llama_eat(struct Animal *self) {

printf("<Llama at %p> Llama's eat grass!\n", (void void*) )(self));

}

/* initialize class */

const struct AnimalClassAnimalVTable Animal = {(void *)NULL 0}; // base class does not implement Animal.Eat

const struct AnimalClassAnimalVTable 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>