Though not enforced, object oriented programming is possible in C. The main constraints added by C++ can be accepted as a convention in the source code. The object oriented programming paradigm is widely used in C projects including the Linux kernel and SQLite.

Classes are composed from two major entity types: data and methods. There is already a tool in C to create structured data types: struct. Simple classes can be translated directly in C:

class person:

   name
   height
   age

can be translated as

struct person {

 char*name;
 int height;
 int age;

}

The methods can be declared as functions, whereas the pointer to the data of the class is put -- by convention -- as first parameter:

class person:

   name
   height
   age
   def set_age(self, n):
       self.age=n

could be translated as

struct person {

 char*name;
 int height;
 int age;

}

void set_age(person * self, int age) {

 self->age=age;

}

methods can also be directly included in the structures

struct person {

 char*name;
 int height;
 int age;
 void (*set_age)(struct person*, int);

}

This allows virtual methods, as the method can be set during the allocation:

void set_age_v1(person * self, int age) {

 self->age=age+1;

}

void set_age_v2(person * self, int age) {

 self->age=age+2;

}

struct person p1; p1->set_age=&set_age_v1;

p1->set_age(p1, 10);