‘Objects’ in C – part 1, the basics

C is on memory allocation quite faster than C++ for C++ contains a lot of overhead due to its object orientated nature.
Although not as convenient in C it is possible to get ‘Object’ like stuff in C as in C++.

I’ll explain the basics in this first part, and continue with more complicated (and neater stuff) in the later parts. I hope you’ll find them usefull.

The base
For this example we’ll use a ‘class’ called ‘example’, first the base:

typedef struct{
	int dummy;
} Example;
Example* ConstructExample(){
	return malloc(sizeof(Example));
}
void DestructExample(Example* example){
	free(example);
}
void main(){
	Example* e;
	e = ConstructExample();
	DestructExample(e);
}

Simple inheritance
Lets add some values, and some simple functions and inheritance. Using the inherited class as the base class is just a simple matter of casting via void*.

typedef struct {
	int a;
} Example;
typedef struct {
	int a;
	int b;
} Example2;
Example2* ConstructExample2() {
	Example2* example = malloc(sizeof(Example2));
	example->b = 10;
	example->a = 11;
	return example;
}
Example* ConstructExample() {
	Example* example = malloc(sizeof(Example));
	example->a = 1;
	return example;
}
void DestructExample(Example* example) {
	free(example);
}
void DestructExample2(Example2* example) {
	free(example);
}
void PrintExample(Example* example) {
	printf("%d ", example->a);
}
void PrintExample2(Example2* example) {
	printf("%d ", example->b);
}
void main() {
	Example* e;
	Example2* e2;
	e = ConstructExample();
	e2 = ConstructExample2();
	PrintExample((void*)e);
	PrintExample((void*)e2);
	PrintExample2((void*)e2);
	DestructExample(e);
	DestructExample2(e2);
}

When inheriting you have to copy the original defenition and only append at the bottom, changing nothing of the previous stuff for otherwise you’ll get nasty errors. When you want to override stuff you got to use neat tricks, more on that lateron.

Virtual functions
Using function pointers, virtual functions can be used:

typedef void (*PrintExample)(void* example);
typedef struct {
	int a;
	PrintExample Print;
} Example;
typedef struct {
	int a;
	PrintExample Print;
	int b;
} Example2;
void PrintExampleImpl(void* example) {
	printf("(printexample) %d", ((Example*)example)->a);
}
void PrintExample2Impl(void* example) {
	printf("(printexample2) %d ", ((Example2*)example)->b);
	PrintExampleImpl(example);
}
Example2* ConstructExample2() {
	Example2* example = malloc(sizeof(Example2));
	example->b = 10;
	example->a = 11;
	example->Print = PrintExample2Impl;
	return example;
}
Example* ConstructExample() {
	Example* example = malloc(sizeof(Example));
	example->a = 1;
	example->Print = PrintExampleImpl;
	return example;
}
void DestructExample(Example* example) {
	free(example);
}
void DestructExample2(Example2* example) {
	free(example);
}
void main() {
	Example* e;
	Example2* e2;
	e = ConstructExample();
	e2 = ConstructExample2();
	e->Print(e);
	e2->Print(e2);
	DestructExample(e);
	DestructExample2(e2);
	getchar();
}

A virtual function still has to be supplied with the function in which it has been called. 2 simple macro can be made to make a this call a bit easier, (for some :p):

#define THISCALLPAR(x,y,z) x->y(x,z)
#define THISCALL(x,y) x->y(x)

Wrapped inheritance
To override some functionality and retain other functionality while adding your own functionality is virtually impossible by using one simple object. Multi-inheritance would be virtually impossible.
2 parts ahead I will talk about these limitations and how to overcome them, the next part discusses inheritance in more detail, and adding ‘Types’ in the mix.

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