oop book programming - Can you write object-oriented code in C?

15 Answers

Since you're talking about polymorphism then yes, you can, we were doing that sort of stuff years before C++ came about.

Basically you use a struct to hold both the data and a list of function pointers to point to the relevant functions for that data.

So, in a communications class, you would have an open, read, write and close call which would be maintained as four function pointers in the structure, alongside the data for an object, something like:

typedef struct {
    int (*close)(void *self);
    // And data goes here.
} tCommClass;

tCommClass commRs232;
commRs232.open = &rs232Open;
: :
commRs232.write = &rs232Write;

tCommClass commTcp;
commTcp.open = &tcpOpen;
: :
commTcp.write = &tcpWrite;

Of course, those code segments above would actually be in a "constructor" such as rs232Init().

When you 'inherit' from that class, you just change the pointers to point to your own functions. Everyone that called those functions would do it through the function pointers, giving you your polymorphism:

int stat = (commTcp.open)(commTcp, "bigiron.box.com:5000");

Sort of like a manual vtable.

You could even have virtual classes by setting the pointers to NULL -the behaviour would be slightly different to C++ (a core dump at run-time rather than an error at compile time).

Here's a piece of sample code that demonstrates it. First the top-level class structure:

#include <stdio.h>

// The top-level class.

typedef struct sCommClass {
    int (*open)(struct sCommClass *self, char *fspec);
} tCommClass;

Then we have the functions for the TCP 'subclass':

// Function for the TCP 'class'.

static int tcpOpen (tCommClass *tcp, char *fspec) {
    printf ("Opening TCP: %s\n", fspec);
    return 0;
static int tcpInit (tCommClass *tcp) {
    tcp->open = &tcpOpen;
    return 0;

And the HTTP one as well:

// Function for the HTTP 'class'.

static int httpOpen (tCommClass *http, char *fspec) {
    printf ("Opening HTTP: %s\n", fspec);
    return 0;
static int httpInit (tCommClass *http) {
    http->open = &httpOpen;
    return 0;

And finally a test program to show it in action:

// Test program.

int main (void) {
    int status;
    tCommClass commTcp, commHttp;

    // Same 'base' class but initialised to different sub-classes.

    tcpInit (&commTcp);
    httpInit (&commHttp);

    // Called in exactly the same manner.

    status = (commTcp.open)(&commTcp, "bigiron.box.com:5000");
    status = (commHttp.open)(&commHttp, "http://www.microsoft.com");

    return 0;

This produces the output:

Opening TCP: bigiron.box.com:5000
Opening HTTP: http://www.microsoft.com

so you can see that the different functions are being called, depending on the sub-class.

tutorial ansi-c pdf

Can you write object-oriented code in C? Especially with regard to polymorphism.

See also question Object-orientation in C.

I believe that besides being useful in its own right, implementing OOP in C is an excellent way to learn OOP and understand its inner workings. Experience of many programmers has shown that to use a technique efficiently and confidently, a programmer must understand how the underlying concepts are ultimately implemented. Emulating classes, inheritance, and polymorphism in C teaches just this.

To answer the original question, here are a couple resources that teach how to do OOP in C:

EmbeddedGurus.com blog post "Object-based programming in C" shows how to implement classes and single inheritance in portable C: http://embeddedgurus.com/state-space/2008/01/object-based-programming-in-c/

Application Note ""C+"—Object Oriented Programming in C" shows how to implement classes, single inheritance, and late binding (polymorphism) in C using preprocessor macros: http://www.state-machine.com/resources/cplus_3.0_manual.pdf, the example code is available from http://www.state-machine.com/resources/cplus_3.0.zip

Sure that is possible. This is what GObject, the framework that all of GTK+ and GNOME is based on, does.

Trivial example with an Animal and Dog: You mirror C++'s vtable mechanism (largely anyway). You also separate allocation and instantiation (Animal_Alloc, Animal_New) so we don't call malloc() multiple times. We must also explicitly pass the this pointer around.

If you were to do non-virtual functions, that's trival. You just don't add them to the vtable and static functions don't require a this pointer. Multiple inheritance generally requires multiple vtables to resolve ambiguities.

Also, you should be able to use setjmp/longjmp to do exception handling.

struct Animal_Vtable{
    typedef void (*Walk_Fun)(struct Animal *a_This);
    typedef struct Animal * (*Dtor_Fun)(struct Animal *a_This);

    Walk_Fun Walk;
    Dtor_Fun Dtor;

struct Animal{
    Animal_Vtable vtable;

    char *Name;

struct Dog{
    Animal_Vtable vtable;

    char *Name; // Mirror member variables for easy access
    char *Type;

void Animal_Walk(struct Animal *a_This){
    printf("Animal (%s) walking\n", a_This->Name);

struct Animal* Animal_Dtor(struct Animal *a_This){
    return a_This;

Animal *Animal_Alloc(){
    return (Animal*)malloc(sizeof(Animal));

Animal *Animal_New(Animal *a_Animal){
    a_Animal->vtable.Walk = Animal_Walk;
    a_Animal->vtable.Dtor = Animal_Dtor;
    a_Animal->Name = "Anonymous";
    return a_Animal;

void Animal_Free(Animal *a_This){


void Dog_Walk(struct Dog *a_This){
    printf("Dog walking %s (%s)\n", a_This->Type, a_This->Name);

Dog* Dog_Dtor(struct Dog *a_This){
    // Explicit call to parent destructor


    return a_This;

Dog *Dog_Alloc(){
    return (Dog*)malloc(sizeof(Dog));

Dog *Dog_New(Dog *a_Dog){
    // Explict call to parent constructor

    a_Dog->Type = "Dog type";
    a_Dog->vtable.Walk = (Animal_Vtable::Walk_Fun) Dog_Walk;
    a_Dog->vtable.Dtor = (Animal_Vtable::Dtor_Fun) Dog_Dtor;

    return a_Dog;

int main(int argc, char **argv){
      Base class:

        Animal *a_Animal = Animal_New(Animal_Alloc());
    Animal *a_Animal = (Animal*)Dog_New(Dog_Alloc());



PS. This is tested on a C++ compiler, but it should be easy to make it work on a C compiler.

This has been interesting to read. I have been pondering the same question myself, and the benefits of thinking about it are this:

  • Trying to imagine how to implement OOP concepts in a non-OOP language helps me understand the strengths of the OOp language (in my case, C++). This helps give me better judgement about whether to use C or C++ for a given type of application -- where the benefits of one out-weighs the other.

  • In my browsing the web for information and opinions on this I found an author who was writing code for an embedded processor and only had a C compiler available: http://www.eetimes.com/discussion/other/4024626/Object-Oriented-C-Creating-Foundation-Classes-Part-1

In his case, analyzing and adapting OOP concepts in plain C was a valid pursuit. It appears he was open to sacrificing some OOP concepts due to the performance overhead hit resulting from attempting to implement them in C.

The lesson I've taken is, yes it can be done to a certain degree, and yes, there are some good reasons to attempt it.

In the end, the machine is twiddling stack pointer bits, making the program counter jump around and calculating memory access operations. From the efficiency standpoint, the fewer of these calculations done by your program, the better... but sometimes we have to pay this tax simply so we can organize our program in a way that makes it least susceptible to human error. The OOP language compiler strives to optimize both aspects. The programmer has to be much more careful implementing these concepts in a language like C.

There are several techniques that can be used. The most important one is more how to split the project. We use an interface in our project that is declared in a .h file and the implementation of the object in a .c file. The important part is that all modules that include the .h file see only an object as a void *, and the .c file is the only module who knows the internals of the structure.

Something like this for a class we name FOO as an example:

In the .h file

#ifndef FOO_H_
#define FOO_H_

 typedef struct FOO_type FOO_type;     /* That's all the rest of the program knows about FOO */

/* Declaration of accessors, functions */
FOO_type *FOO_new(void);
void FOO_free(FOO_type *this);
void FOO_dosomething(FOO_type *this, param ...):
char *FOO_getName(FOO_type *this, etc);

The C implementation file will be something like that.

#include <stdlib.h>
#include "FOO.h"

struct FOO_type {

FOO_type *FOO_new(void)
    FOO_type *this = calloc(1, sizeof (FOO_type));

    FOO_dosomething(this, );
    return this;

So I give the pointer explicitly to an object to every function of that module. A C++ compiler does it implicitly, and in C we write it explicitly out.

I really use this in my programs, to make sure that my program does not compile in C++, and it has the fine property of being in another color in my syntax highlighting editor.

The fields of the FOO_struct can be modified in one module and another module doesn't even need to be recompiled to be still usable.

With that style I already handle a big part of the advantages of OOP (data encapsulation). By using function pointers, it's even easy to implement something like inheritance, but honestly, it's really only rarely useful.

If you are convinced that an OOP approach is superior for the problem you are trying to solve, why would you be trying to solve it with a non-OOP language? It seems like you're using the wrong tool for the job. Use C++ or some other object-oriented C variant language.

If you are asking because you are starting to code on an already existing large project written in C, then you shouldn't try to force your own (or anyone else's) OOP paradigms into the project's infrastructure. Follow the guidelines that are already present in the project. In general, clean APIs and isolated libraries and modules will go a long way towards having a clean OOP-ish design.

If, after all this, you really are set on doing OOP C, read this (PDF).

Yes, you can. People were writing object-oriented C before C++ or Objective-C came on the scene. Both C++ and Objective-C were, in parts, attempts to take some of the OO concepts used in C and formalize them as part of the language.

Here's a really simple program that shows how you can make something that looks-like/is a method call (there are better ways to do this. This is just proof the language supports the concepts):


struct foobarbaz{
    int one;
    int two;
    int three;
    int (*exampleMethod)(int, int);

int addTwoNumbers(int a, int b){
    return a+b;

int main()
    // Define the function pointer
    int (*pointerToFunction)(int, int) = addTwoNumbers;

    // Let's make sure we can call the pointer
    int test = (*pointerToFunction)(12,12);
    printf ("test: %u \n",  test);

    // Now, define an instance of our struct
    // and add some default values.
    struct foobarbaz fbb;
    fbb.one   = 1;
    fbb.two   = 2;
    fbb.three = 3;

    // Now add a "method"
    fbb.exampleMethod = addTwoNumbers;

    // Try calling the method
    int test2 = fbb.exampleMethod(13,36);
    printf ("test2: %u \n",  test2);

    return 0;

A little OOC code to add:

#include <stdio.h>

struct Node {
    int somevar;

void print() {
    printf("Hello from an object-oriented C method!");

struct Tree {
    struct Node * NIL;
    void (*FPprint)(void);
    struct Node *root;
    struct Node NIL_t;
} TreeA = {&TreeA.NIL_t,print};

int main()
    struct Tree TreeB;
    TreeB = TreeA;
    return 0;

OOP is only a paradigm which place datas as more important than code in programs. OOP is not a language. So, like plain C is a simple language, OOP in plain C is simple too.

The answer to the question is 'Yes, you can'.

Object-oriented C (OOC) kit is for those who want to program in an object-oriented manner, but sticks on the good old C as well. OOC implements classes, single and multiple inheritance, exception handling.


• Uses only C macros and functions, no language extensions required! (ANSI-C)

• Easy-to-read source code for your application. Care was taken to make things as simple as possible.

• Single inheritance of classes

• Multiple inheritance by interfaces and mixins (since version 1.3)

• Implementing exceptions (in pure C!)

• Virtual functions for classes

• External tool for easy class implementation

For more details, visit http://ooc-coding.sourceforge.net/.

See http://slkpg.byethost7.com/instance.html for yet another twist on OOP in C. It emphasizes instance data for reentrancy using just native C. Multiple inheritance is done manually using function wrappers. Type safety is maintained. Here is a small sample:

typedef struct _peeker
    log_t     *log;
    symbols_t *sym;
    scanner_t  scan;            // inherited instance
    peek_t     pk;
    int        trace;

    void    (*push) ( SELF *d, symbol_t *symbol );
    short   (*peek) ( SELF *d, int level );
    short   (*get)  ( SELF *d );
    int     (*get_line_number) ( SELF *d );

} peeker_t, SlkToken;

#define push(self,a)            (*self).push(self, a)
#define peek(self,a)            (*self).peek(self, a)
#define get(self)               (*self).get(self)
#define get_line_number(self)   (*self).get_line_number(self)

(get_line_number) ( peeker_t *d )
    return  d->scan.line_number;

InitializePeeker ( peeker_t  *peeker,
                   int        trace,
                   symbols_t *symbols,
                   log_t     *log,
                   list_t    *list )
    InitializeScanner ( &peeker->scan, trace, symbols, log, list );
    peeker->log = log;
    peeker->sym = symbols;
    peeker->pk.current = peeker->pk.buffer;
    peeker->pk.count = 0;
    peeker->trace = trace;

    peeker->get_line_number = get_line_number;
    peeker->push = push;
    peeker->get = get;
    peeker->peek = peek;

I built a little library where I tried that and to me it works real nicely. So I thought I share the experience.


Single inheritance can be implemented quite easily using a struct and extending it for every other child class. A simple cast to the parent structure makes it possible to use parent methods on all the descendants. As long as you know that a variable points to a struct holding this kind of an object you can always cast to the root class and do introspection.

As has been mentioned, virtual methods are somewhat trickier. But they are doable. To keep things simple I just use an array of functions in the class description structure which every child class copies and repopulates individual slots where required.

Multiple inheritance would be rather complicated to implement and comes with a significant performance impact. So I leave it. I do consider it desirable and useful in quite a few cases to cleanly model real life circumstances, but in probably 90% of cases single inheritance covers the needs. And single inheritance is simple and costs nothing.

Also I do not care about type safety. I think you should not depend on the compiler to prevent you from programming mistakes. And it shields you only from a rather small part of errors anyway.

Typically, in an object oriented environment you also want to implement reference counting to automate memory management to the extent possible. So I also put a reference count into the “Object” root class and some functionality to encapsulate allocation and deallocation of heap memory.

It is all very simple and lean and gives me the essentials of OO without forcing me to deal with the monster that is C++. And I retain the flexibility of staying in C land, which among other things makes it easier to integrate third party libraries.

Yes, but I have never seen anyone attempt to implement any sort of polymorphism with C.



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