The difference between class and object.

 The difference between class and object is subtle and misunderstood frequently. It is important to understand the difference in order to write cleaner class heirachies. 

First, the word "Object" is taken from Philosophy that is used to describe something that exists in the world around you at this very minute in time. For example; the computer in front of you, the keyboard you are using and yourself. 

Secondly, classes represent groups of these objects. A class called Keyboard represents all keyboard objects.

Class keyboard and its objects

 

Hopefully this has shown you the difference between an object and a class in terms of philosophy so that you know what should be a class and what shouldn't.

Single inheritance in C

Inheritance is a powerful mechanism for code organization, unfortunatly the class concept is not present in the C language. Therefore, there is a need for the inheritance mechanism in C where C++ is not available. Inheritance is an extremely complex mechanism, we can only hope to capture a small slice of its features in C without breaking the C language too much. The solution is to make two structures compatible if a pointer to the derived type is upcasted to the base type, therefore we can use the derived type in the base type's procedures.

 

typedef struct { 

    float originX, originY;

} Shape;

void SetOrigin(Shape* shape, float x, float y); 

float GetOriginX(Shape* shape); 

float GetOriginY(Shape* shape);

 

typedef struct { 

    Shape base; // this allows Circle* to represent Shape*

    float radius;

} Circle; 

void SetRadius(Circle* circle);