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Classes in C

This document describes the simplest possible coding style for making classes in C. It will describe constructors, instance variables, instance methods, class variables, class methods, inheritance, polymorphism, namespaces with aliasing and put it all together in an example project.

  1. C Classes
  2. Constructors
  3. Methods
  4. Inheritance
  5. Controlling Access to Members
  6. Abstract Classes, Abstract Methods and Interfaces
  7. Namespaces
  8. An Example Project

1. C Classes

A class consists of an instance type and a class object:

An instance type is a struct containing variable members called instance variables and function members called instance methods. A variable of the instance type is called an instance.

A class object is a global const struct variable containing class variables and class methods. These members belong the whole class without any references to any instances.

A class named "Complex" should name the instance type struct Complex and the class object Complex, and put the interface definitions in "Complex.h" and the implementation in "Complex.c".

		struct Complex {
		extern const struct ComplexClass {
		} Complex;
		#include "Complex.h"
		const struct ComplexClass Complex={...};


2. Constructors

Instances must be initialized by constructors when declared, and the constructors must be class methods. The constructor should preferably return an instance type, but may also return a pointer to an instance type.

The Complex class gets two instance variables re and im, and a constructor named new:

		struct Complex {
			double re, im;
		extern const struct ComplexClass {
			struct Complex (*new)(double real, double imag);
		} Complex;
		#include "Complex.h"
		static struct Complex new(double real, double imag) {
			return (struct Complex){.re=real, .im=imag};
		const struct ComplexClass Complex={.new=&new};
		struct Complex c=Complex.new(3., -4.);

3. Methods

Instance methods must be declared as instance type members pointing to the wanted function prototype, and that pointer must be set by the constructor. Typically, the method pointer is set to a static function defined in the implementation file.

To be able to access the instance's data, instance methods must receive a pointer to the instance as its first argument. This argument is typically named this.

When we add an instance method abs() for calculating the absolute value of a complex number we get:

		struct Complex {
			double re, im;
			double (*abs)(struct Complex *this);
		extern const struct ComplexClass {
			struct Complex (*new)(double real, double imag);
		} Complex;
		#include "Complex.h"
		static double abs(struct Complex *this) {
			return sqrt(this->re*this->re+this->im*this->im);
		static struct Complex new(double real, double imag) {
			return (struct Complex){.re=real, .im=imag, .abs=&abs};
		const struct ComplexClass Complex={.new=&new};
		struct Complex c=Complex.new(3., -4.);
		printf("%g\n", c.abs(&c)); // Prints 5

Class methods must be initialized in the same way as instance methods, but have no restriction on the prototype.

4. Inheritance

A base class must be represented as a member variable with the same name and type as the base class itself.

A subclass may override the base class instance method pointers to provide polymorphism. The subclass must override with an identically prototyped function and set the base class' method pointer in the constructor after the baseclass' constructor has been called.

Whenever an overriden instance method is called, we are guaranteed that it was called by an instance of the baseclass. Since the instance method receives a pointer to the base class as its first argument, we may get the subclass using the offsetof() macro from stddef.h.

The following files shows a simple example of inheritance and polymorphism:


	struct Employee {
		const char *first_name;
		const char *family_name;
		const char *(*print)(struct Employee *this,
				     size_t bufsize,char buf[bufsize]);
	extern const struct EmployeeClass {
		struct Employee (*new)(const char *first_name,
				       const char *family_name);
	} Employee;



	#include "Employee.h"
	struct Manager {
		struct Employee Employee;
		int level;
	extern const struct ManagerClass {
		struct Manager (*new)(const char *first_name,
				      const char *family_name, int level);
	} Manager;



	#include "Manager.h"
	int main(void)
		struct Manager manager=Manager.new("Håkon", "Hallingstad", 3);
		struct Employee employee=Employee.new("Håkon", "Hallingstad");
		struct Employee *polymorph=&manager.Employee;
		char buf[50];
		printf("%s\n", employee.print(&employee, sizeof(buf), buf));
		printf("%s\n", polymorph->print(polymorph, sizeof(buf), buf));
		return 0;

The Manager class overrides Employee's print() instance method with the line from Manager.c:


Which makes inheritance.c print:

	Name: Håkon Hallingstad
	Name: Håkon Hallingstad, level 3

5. Controlling Access to Members

In object oriented languages each member has an access attribute and the compiler will enforce that access attribute.

With Classes in C we should use comments to specify the access attributes. For instance we use the following notation:

	struct Complex {
		// protected:
		// private:

6. Abstract Classes, Abstract Methods and Interfaces

In object oriented languages we can specify an abstract class to guarantee that the class cannot be instanciated. Abstract methods and interfaces can be used to guarantee that subclasses override methods.

With Classes in C just have to make sure any user of the class understands such intensions, for instance:

	struct ElementInterface {

	/*interface*/ struct Element {

	/*abstract*/ struct Complex {

	struct Stack {
		/*abstract*/ double (*foo)(struct Stack *this);

Abstract instance method pointers should be initialized to NULL.

7. Namespaces

A namespace defines a common prefix of all identifiers exported by a class and the path of its header- and implementation- files.

For instance a Complex class with a namespace org_pvv_hakonhal_utils_Complex should have its implementation file in org/pvv/hakonhal/utils/Complex.c, and its header file in org/pvv/hakonhal/utils/Complex.h containing:

	struct org_pvv_hakonhal_utils_Complex {
	extern struct org_pvv_hakonhal_utils_ComplexClass {
	} org_pvv_hakonhal_utils_Complex;

When we are going to use the class we may alias the identifiers to make them more managable, by using the #define directive:

	#include "org/pvv/hakonhal/utils/Complex.h"
	#define Complex org_pvv_hakonhal_utils_Complex
	struct Complex c=Complex.new();

8. An Example Project

In this example project we will create and test a bounds-checking stack implementation by extending a simpler stack implementation. The project will illustrate everything about C Classes including constructors, methods, inheritance, namespaces and aliases.

For compiling this project you should read C Project Building.

The Libray Project

We imagine the simple stack project has been downloaded from the net and the header file may be referenced as org/somewhere/someone/Stack.h. The stack header contains:


	struct org_somewhere_someone_Stack_ElementI {
	struct org_somewhere_someone_Stack {
		void (*push)
		(struct org_somewhere_someone_Stack *this,
		 struct org_somewhere_someone_Stack_ElementI *element);
		struct org_somewhere_someone_Stack_ElementI *(*pop)
		(struct org_somewhere_someone_Stack *this);
		// protected:
		int count;
		struct org_somewhere_someone_Stack_ElementI *
	extern struct org_somewhere_someone_StackClass {
		struct org_somewhere_someone_Stack (*new)(void);
	} org_somewhere_someone_Stack;

org_somewhere_someone_Stack_ElementI is an interface for the elements that is stored in the stack, and impose no restrictions on the elements stored since the struct has an empty body.

We choose a unique org_pvv_hakonhal_utils namespace and creates the org/pvv/hakonhal/utils directory where we will put our BStack.c and BStack.h files.

The only thing our class will do in addition to org_somewhere_someone_Stack, is to check the bounds when push()'ing and pop()'ing, so our instance type only holds the reference to the base class:


	#include "org/somewhere/someone/Stack.h"
	struct org_pvv_hakonhal_utils_BStack {
		struct org_somewhere_someone_Stack org_somewhere_someone_Stack;
	extern const struct org_pvv_hakonhal_utils_BStackClass {
		struct org_pvv_hakonhal_utils_BStack (*new)(void);
	} org_pvv_hakonhal_utils_BStack;

The implementation file is somewhat more complex.


1	#include "org/pvv/hakonhal/utils/BStack.h"
2	#define BStack      org_pvv_hakonhal_utils_BStack
3	#define BStackClass org_pvv_hakonhal_utils_BStackClass
4	#include "org/somewhere/someone/Stack.h"
5	#define ElementI    org_somewhere_someone_Stack_ElementI
6	#define Stack       org_somewhere_someone_Stack
8	#include <stdio.h>
9	#include <stdlib.h>
10	static void (*base_push)(struct Stack *this, struct ElementI *element);
11	static void push(struct Stack *base, struct ElementI *element)
12	{
13		if (base->count>=STACK_SIZE) {
14			fprintf(stderr, "%s", "Stack overflow!\n");
15			exit(1);
16		}
18		base_push(base, element);
19	}
20	static struct ElementI *(*base_pop)(struct Stack *this);
21	static struct ElementI *pop(struct Stack *base)
22	{
23		if (base->count<=0) {
24			fprintf(stderr, "%s", "Stack underflow!\n");
25			exit(1);
26		}
27		return base_pop(base);
28	}
29	static struct BStack new(void)
30	{
31		struct BStack ret;
32		ret.Stack=Stack.new();
33		base_push=ret.Stack.push;
34		ret.Stack.push=&push;
35		base_pop=ret.Stack.pop;
36		ret.Stack.pop=&pop;
37		return ret;
38	}
39	const struct BStackClass BStack={.new=&new};

Let us look at the implementation of the push() instance method, the pop() is similar. Since it is using the base class' push() instance method, we must keep a pointer to the base class' instance method, see 10, 18 and 33-34.

Testing the Library Project

The implementation of the Bstack class is rather long:


1	#include "org/pvv/hakonhal/utils/BStack.h"
2	#define BStack org_pvv_hakonhal_utils_BStack
3	#include "org/somewhere/someone/Stack.h"
4	#define ElementI org_somewhere_someone_Stack_ElementI
5	#define Stack org_somewhere_someone_Stack
6	#include <stddef.h>
7	#include <stdio.h>
8	struct Integer {
9		struct ElementI ElementI;
10		int value;
11		void (*print)(struct Integer *this, const char *id);
12	};
13	static void print(struct Integer *this, const char *id)
14	{
15		printf("%s: %d\n", id, this->value);
16	}
17	static struct Integer new(int value)
18	{
19		return (struct Integer){
20			.ElementI={},
21			.value=value,
22			.print=&print,
23		};
24	}
25	static const struct {
26		struct Integer (*new)(int value);
27	} Integer={.new=&new};
28	int main(void)
29	{
30		struct BStack stack=BStack.new();
31		struct Integer i=Integer.new(10), j=Integer.new(20);
32		struct Integer *ptr;
33		stack.Stack.push(&stack.Stack, &i.ElementI);
34		stack.Stack.push(&stack.Stack, &j.ElementI);
35		ptr=(void *)stack.Stack.pop(&stack.Stack)-offsetof(struct Integer, ElementI);
36		ptr->print(ptr, "j");
37		ptr=((void *)stack.Stack.pop(&stack.Stack))-offsetof(struct Integer, ElementI);
38		ptr->print(ptr, "i");
39		printf("%s\n", "Will now try to pop an empty stack");
40		stack.Stack.pop(&stack.Stack);
41		return 0;
42	}

We define an Integer class that extends the org_somewhere_someone_Stack_ElementI interface so that it might be added to our BStack, see lines 8-27. The class also contains an int and an instance method to print it. Note that since we are constructing an executable, the Integer class does not need to have a namespace.

Since we are using aliasing, statements such as that on line 30 actually reads:

	struct org_pvv_hakonhal_utils_BStack stack=org_pvv_hakonhal_utils_BStack.new();

Since the push() and pop() instance methods where defined by Stack, we need to "go through" the Stack subclass to call them, as seen on e.g. line 33-34.

When we retreive the elements on the stack, we need the slightly awkward syntax in 35 and 37. Conceptually, we receive a pointer to the ElementI base class of a struct BStack variable, so we just need to shift it.

The BStack_test executable will output:

	j: 20
	i: 10
	Will now try to pop an empty stack
	Stack underflow detected!
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Last modified: Mon Jun 21 09:00:46 CEST 2004