Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 6 years ago.
Improve this question
What are some ways to write object-oriented code in C? Especially with regard to polymorphism.
See also this Stack Overflow question Object-orientation in C.
Yes. In fact Axel Schreiner provides his book "Object-oriented Programming in ANSI-C" for free which covers the subject quite thoroughly.
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 (*open)(void *self, char *fspec);
int (*close)(void *self);
int (*read)(void *self, void *buff, size_t max_sz, size_t *p_act_sz);
int (*write)(void *self, void *buff, size_t max_sz, size_t *p_act_sz);
// 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.
Namespaces are often done by doing:
stack_push(thing *)
instead of
stack::push(thing *)
To make a C struct into something like a C++ class you can turn:
class stack {
public:
stack();
void push(thing *);
thing * pop();
static int this_is_here_as_an_example_only;
private:
...
};
Into
struct stack {
struct stack_type * my_type;
// Put the stuff that you put after private: here
};
struct stack_type {
void (* construct)(struct stack * this); // This takes uninitialized memory
struct stack * (* operator_new)(); // This allocates a new struct, passes it to construct, and then returns it
void (*push)(struct stack * this, thing * t); // Pushing t onto this stack
thing * (*pop)(struct stack * this); // Pops the top thing off the stack and returns it
int this_is_here_as_an_example_only;
}Stack = {
.construct = stack_construct,
.operator_new = stack_operator_new,
.push = stack_push,
.pop = stack_pop
};
// All of these functions are assumed to be defined somewhere else
And do:
struct stack * st = Stack.operator_new(); // Make a new stack
if (!st) {
// Do something about it
} else {
// You can use the stack
stack_push(st, thing0); // This is a non-virtual call
Stack.push(st, thing1); // This is like casting *st to a Stack (which it already is) and doing the push
st->my_type.push(st, thing2); // This is a virtual call
}
I didn't do the destructor or delete, but it follows the same pattern.
this_is_here_as_an_example_only is like a static class variable -- shared among all instances of a type. All methods are really static, except that some take a this *
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
I've seen it done. I wouldn't recommend it. C++ originally started this way as a preprocessor that produced C code as an intermediate step.
Essentially what you end up doing is create a dispatch table for all of your methods where you store your function references. Deriving a class would entail copying this dispatch table and replacing the entries that you wanted to override, with your new "methods" having to call the original method if it wants to invoke the base method. Eventually, you end up rewriting C++.
Sure that is possible. This is what GObject, the framework that all of GTK+ and GNOME is based on, does.
The C stdio FILE sub-library is an excellent example of how to create abstraction, encapsulation, and modularity in unadulterated C.
Inheritance and polymorphism - the other aspects often considered essential to OOP - do not necessarily provide the productivity gains they promise and reasonable arguments have been made that they can actually hinder development and thinking about the problem domain.
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){
printf("animal::dtor\n");
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){
a_This->vtable.Dtor(a_This);
free(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
Animal_Dtor((Animal*)a_This);
printf("dog::dtor\n");
return a_This;
}
Dog *Dog_Alloc(){
return (Dog*)malloc(sizeof(Dog));
}
Dog *Dog_New(Dog *a_Dog){
// Explict call to parent constructor
Animal_New((Animal*)a_Dog);
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());
a_Animal->vtable.Walk(a_Animal);
Animal_Free(a_Animal);
}
PS. This is tested on a C++ compiler, but it should be easy to make it work on a C compiler.
Check out GObject. It's meant to be OO in C and one implementation of what you're looking for. If you really want OO though, go with C++ or some other OOP language. GObject can be really tough to work with at times if you're used to dealing with OO languages, but like anything, you'll get used to the conventions and flow.
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);
#endif
The C implementation file will be something like that.
#include <stdlib.h>
...
#include "FOO.h"
struct FOO_type {
whatever...
};
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.
You may find it helpful to look at Apple's documentation for its Core Foundation set of APIs. It is a pure C API, but many of the types are bridged to Objective-C object equivalents.
You may also find it helpful to look at the design of Objective-C itself. It's a bit different from C++ in that the object system is defined in terms of C functions, e.g. objc_msg_send to call a method on an object. The compiler translates the square bracket syntax into those function calls, so you don't have to know it, but considering your question you may find it useful to learn how it works under the hood.
You can fake it using function pointers, and in fact, I think it is theoretically possible to compile C++ programs into C.
However, it rarely makes sense to force a paradigm on a language rather than to pick a language that uses a paradigm.
Object oriented C, can be done, I've seen that type of code in production in Korea, and it was the most horrible monster I'd seen in years (this was like last year(2007) that I saw the code).
So yes it can be done, and yes people have done it before, and still do it even in this day and age. But I'd recommend C++ or Objective-C, both are languages born from C, with the purpose of providing object orientation with different paradigms.
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):
#include<stdio.h>
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);
printf("\nDone\n");
return 0;
}
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).
Of course, it just won't be as pretty as using a language with built-in support. I've even written "object-oriented assembler".
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;
TreeB.FPprint();
return 0;
}
I've been digging this for one year:
As the GObject system is hard to use with pure C, I tried to write some nice macros to ease the OO style with C.
#include "OOStd.h"
CLASS(Animal) {
char *name;
STATIC(Animal);
vFn talk;
};
static int Animal_load(Animal *THIS,void *name) {
THIS->name = name;
return 0;
}
ASM(Animal, Animal_load, NULL, NULL, NULL)
CLASS_EX(Cat,Animal) {
STATIC_EX(Cat, Animal);
};
static void Meow(Animal *THIS){
printf("Meow!My name is %s!\n", THIS->name);
}
static int Cat_loadSt(StAnimal *THIS, void *PARAM){
THIS->talk = (void *)Meow;
return 0;
}
ASM_EX(Cat,Animal, NULL, NULL, Cat_loadSt, NULL)
CLASS_EX(Dog,Animal){
STATIC_EX(Dog, Animal);
};
static void Woof(Animal *THIS){
printf("Woof!My name is %s!\n", THIS->name);
}
static int Dog_loadSt(StAnimal *THIS, void *PARAM) {
THIS->talk = (void *)Woof;
return 0;
}
ASM_EX(Dog, Animal, NULL, NULL, Dog_loadSt, NULL)
int main(){
Animal *animals[4000];
StAnimal *f;
int i = 0;
for (i=0; i<4000; i++)
{
if(i%2==0)
animals[i] = NEW(Dog,"Jack");
else
animals[i] = NEW(Cat,"Lily");
};
f = ST(animals[0]);
for(i=0; i<4000; ++i) {
f->talk(animals[i]);
}
for (i=0; i<4000; ++i) {
DELETE0(animals[i]);
}
return 0;
}
Here is my project site (I don't have enough time to write en. doc,however the doc in chinese is much better).
OOC-GCC
There is an example of inheritance using C in Jim Larson's 1996 talk given at the Section 312 Programming Lunchtime Seminar here: High and Low-Level C.
Which articles or books are good to use OOP concepts in C?
Dave Hanson's C Interfaces and Implementations is excellent on encapsulation and naming and very good on use of function pointers. Dave does not try to simulate inheritance.
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.
One thing you might want to do is look into the implementation of the Xt toolkit for X Window. Sure it is getting long in the tooth, but many of the structures used were designed to work in an OO fashion within traditional C. Generally this means adding an extra layer of indirection here and there and designing structures to lay over each other.
You can really do lots in the way of OO situated in C this way, even though it feels like it some times, OO concepts did not spring fully formed from the mind of #include<favorite_OO_Guru.h>. They really constituted many of the established best practice of the time. OO languages and systems only distilled and amplified parts of the programing zeitgeist of the day.
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.
Features
• 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/.
It's seem like people are trying emulate the C++ style using C. My take is that doing object-oriented programming C is really doing struct-oriented programming. However, you can achieve things like late binding, encapsulation, and inheritance. For inheritance you explicitly define a pointer to the base structs in your sub struct and this is obviously a form of multiple inheritance. You'll also need to determine if your
//private_class.h
struct private_class;
extern struct private_class * new_private_class();
extern int ret_a_value(struct private_class *, int a, int b);
extern void delete_private_class(struct private_class *);
void (*late_bind_function)(struct private_class *p);
//private_class.c
struct inherited_class_1;
struct inherited_class_2;
struct private_class {
int a;
int b;
struct inherited_class_1 *p1;
struct inherited_class_2 *p2;
};
struct inherited_class_1 * new_inherited_class_1();
struct inherited_class_2 * new_inherited_class_2();
struct private_class * new_private_class() {
struct private_class *p;
p = (struct private_class*) malloc(sizeof(struct private_class));
p->a = 0;
p->b = 0;
p->p1 = new_inherited_class_1();
p->p2 = new_inherited_class_2();
return p;
}
int ret_a_value(struct private_class *p, int a, int b) {
return p->a + p->b + a + b;
}
void delete_private_class(struct private_class *p) {
//release any resources
//call delete methods for inherited classes
free(p);
}
//main.c
struct private_class *p;
p = new_private_class();
late_bind_function = &implementation_function;
delete_private_class(p);
compile with c_compiler main.c inherited_class_1.obj inherited_class_2.obj private_class.obj.
So the advice is to stick to a pure C style and not try to force into a C++ style. Also this way lends itself to a very clean way of building an API.
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)
INSTANCE_METHOD
int
(get_line_number) ( peeker_t *d )
{
return d->scan.line_number;
}
PUBLIC
void
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'm a bit late to the party, but I want to share my experience on the topic: I work with embedded stuff these days, and the only (reliable) compiler I have is C, so that I want to apply object-oriented approach in my embedded projects written in C.
Most of the solutions I've seen so far use typecasts heavily, so we lose type safety: compiler won't help you if you make a mistake. This is completely unacceptable.
Requirements that I have:
Avoid typecasts as much as possible, so we don't lose type safety;
Polymorphism: we should be able to use virtual methods, and user of the class should not be aware whether some particular method is virtual or not;
Multiple inheritance: I don't use it often, but sometimes I really want some class to implement multiple interfaces (or to extend multiple superclasses).
I've explained my approach in detail in this article: Object-oriented programming in C; plus, there is an utility for autogeneration of boilerplate code for base and derived classes.
I built a little library where I tried that and to me it works real nicely. So I thought I share the experience.
https://github.com/thomasfuhringer/oxygen
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.
I propose to use Objective-C, which is a superset of C.
While Objective-C is 30 years old, it allows to write elegant code.
http://en.wikipedia.org/wiki/Objective-C
Yes, it is possible.
This is pure C, no macros preprocessing. It has inheritance, polymorphism, data encapsulation (including private data). It does not have equivalent protected qualifier, wich means private data is private down the inheritance chain too.
#include "triangle.h"
#include "rectangle.h"
#include "polygon.h"
#include <stdio.h>
int main()
{
Triangle tr1= CTriangle->new();
Rectangle rc1= CRectangle->new();
tr1->width= rc1->width= 3.2;
tr1->height= rc1->height= 4.1;
CPolygon->printArea((Polygon)tr1);
printf("\n");
CPolygon->printArea((Polygon)rc1);
}
/*output:
6.56
13.12
*/
I am trying to understand the existing code.
When do we actually go for function pointers? specially like the one below.
struct xx
{
char *a;
(*func)(char *a, void *b);
void *b;
}
struct xx ppp[] = { };
then check sizeof(ppp)/sizeof(*ppp);
when do we go with such kind of approach?
sizeof array / sizeof *array is a way of finding out how many elements are in an array. (Note that it must be an array rather than a pointer.) I'm not sure how that's related to your function pointer question.
Function pointers are used to store a reference to a function so that it can be called later. The key thing is that a function pointer needn't always point to the same function. (If it did, you could just refer to the function by name.)
Here's an example based on your code (although I could provide a better one if I knew what your code was supposed to do.
char *s1 = "String one";
char *s2 = "String two";
void f(char *a, void *b) {
/* Do something with a and b */
}
void g(char *a, void *b) {
/* Do something else with a and b */
}
struct xx {
char *a;
void (*func)(char *a, void *b);
void *b;
}
struct xx ppp[] = { {s1, f, NULL}, {s2, g, NULL} };
int main(int argc, char **argv) {
for (int i = 0; i < (sizeof ppp / sizeof *ppp); i++) {
ppp[i].func(ppp[i].a, ppp[i].b);
}
}
There are two major uses (that I know of) for function pointers in C.
1. Callbacks
You have some sort of event-driven framework (a GUI is one of the easiest examples), and the program wants to react to events as they happen. Now you can do that with an event pump, like
while (event *e = get_one_event()) {
switch (e->type) {
case EVT_CLICK:
...
}
}
but that gets tiring after a while. The other major alternative is callbacks. The program defines a bunch of functions to handle different events, and then registers them with the library: "when event X happens, call function Y" -- so of course, the library is going to receive a function pointer, and call it at the relevant time.
2. Objects (function tables / vtables)
If you've done OO in most other languages, this should be fairly easy for you to picture. Imagine an object as a struct that contains its members and then a bunch of function pointers (or, maybe more likely, its members and a pointer to another struct representing its class, that contains a bunch of function pointers). The function pointers in the table are the object's methods. GLib/GObject is a big user of this technique, as is the Linux kernel (struct file_operations, struct device_driver, struct bus_type, and many many more). This lets us have an arbitrary number of objects with different behavior, without multiplying the amount of code.
When do we actually go for function pointers? specially like the one below.
You use function pointer when you want to make something more abstract.
By example, suppose your application has a graphical toolbox with a certain number of buttons. Every button corresponds to an instance of a certain struct.
The button structure can contain a function pointer and a context:
typedef struct {
void (*press_button) (void *context);
void *context;
/* Here some other stuff */
} Button;
When the user clicks the button, the event is something like
void event_click (Button *b)
{
b->press_button(b->context);
}
The point in doing this is that you can use always the same structure for each button:
Button * create_button (void (*callback) (void *), void *context, /* other params */
{
Button *ret = malloc(sizeof(Button));
if (ret != NULL) {
ret->callback = callback;
ret->context = context;
/* Assign other params */
}
...
return ret;
}
So when you build your toolbox you probably do something like
Button * toolbox[N];
toolbox[0] = create_button(function1, (void *)data, ...);
toolbox[1] = create_button(function2, (void *)something, ...);
...
toolbox[N-1] = create_button(functionN, (void *)something_else, ...);
Also when you create some function pointer, always carry some contxt information (like I did with the context field of the struct). This allows you to avoid global variables, thus you can get a robust and reentrant code!
Note:
This method is awesome, but if you deal with C++ you may prefer to use object orientation and replace callbacks with derivaton from abstract classes. By doing this you also don't need to carry the context, since the class will do it for you.
Edit in answer of first comment:
The current code I am going through is related to file IO. setting an environment variable and creating symbolic links between files, copying data from one file to another, etc. I am not understanding why do we need to call these functions at run time using function pointers. we can as well call them directly.
In fact you can do what you need without using function pointers. If I do understand well your problem, you are trying to understand someone else's code, which is doing what you listed with function pointers.
Personally I don't use this feature unless I need it but if you post here some additional code maybe we can try to understand it better.
gcc 4.4.3 c89
I am creating a client server application and I will need to implement some callback functions.
However, I am not too experienced in callbacks. And I am wondering if anyone knowns some good reference material to follow when designing callbacks. Is there any design patterns that are used for c. I did look at some patterns but there where all c++.
Many thanks for any suggestions,
Here is a very rough example. Please note, the only thing I'm trying to demonstrate is the use of callbacks, its designed to be informational, not a demonstration.
Lets say that we have a library (or any set of functions that revolve around a structure), we're going to have code that looks similar to this (of course, I'm naming it foo):
typedef struct foo {
int value;
char *text;
} foo_t;
That's simple enough. We'd then (conventionally) provide some means of allocating and freeing it, such as:
foo_t *foo_start(void)
{
foo_t *ret = NULL;
ret = (foo_t *)malloc(sizeof(struct foo));
if (ret == NULL)
return NULL;
return ret;
}
And then:
void foo_stop(foo_t *f)
{
if (f != NULL)
free(f);
}
But we want a callback, so we can define a function that will be entered when foo->text has something to report. To do that, we use a typed function pointer:
typedef void (* foo_callback_t)(int level, const char *data);
We also want any of the foo family of functions to be able to enter this callback conveniently. To do that, we need to add it to the structure, which would now look like this:
typedef struct foo {
int value;
char *text;
foo_callback_t callback;
} foo_t;
Then we write the function that will actually be entered (using the same prototype of our callback type):
void my_foo_callback(int val, char *data)
{
printf("Val is %d, data is %s\n", val, data == NULL ? "NULL" : data);
}
We then need to write some convenient way to say what function it actually points to:
void foo_reg_callback(foo_t *f, void *cbfunc)
{
f->callback = cbfunc;
}
And then our other foo functions can use it, for instance:
int foo_bar(foo_t *f, char *data)
{
if (data == NULL)
f->callback(LOG_ERROR, "data was NULL");
}
Note that in the above:
f->callback(LOG_ERROR, "data was NULL");
Is just like doing this:
my_foo_callback(LOG_ERROR, "data was NULL"):
Except that, we enter my_foo_callback() via a function pointer that we previously set, thereby giving us the flexibility to define our own handler on the fly (and even switch handlers if / as needed).
One of the biggest problems with callbacks (and even the code above) is type safety when using them. A lot of callbacks will take a void * pointer, usually named something like context which could be any type of data/memory. This provides great flexibility, but can be problematic if your pointers get away from you. For instance, you don't want to accidentally cast what is actually a struct * as char * (or int for that matter) by assignment. You can pass much more than simple strings and integers - structures, unions, enums, etc can all be passed. CCAN's type safe callbacks help you to avoid unwittingly evil casts (to / from void *) when doing so.
Again, this is an over simplified example that's designed to give you an overview of one possible way to use callbacks. Please consider it psuedo code that is meant only as an example.
IN C, callbacks are done with function pointers.
One feature that you definitely want is user defined context. Your code takes a void * pointer and makes it available to the callback function:
void callback(..., void *ctx);
void call_service_which_invokes_callback(...,
void (*cb)(..., void *ctx),
void *ctx);
This way, the callback can access any necessary state without having to use global variables.
Callbacks in C are implemented using function pointers. This might be helpful for starting points:
What is a "callback" in C and how are they implemented?
Also,
http://www.newty.de/fpt/callback.html#howto
Could you suggest a syntax for the C language to use it in a similar way than an object-oriented language? I know that they cannot be the same and that some keywords aren't present in C, but I'm wondering if there is a way to take advantage of certain aspects (like inheritance) even in a C program.
You can implement polymorphism with regular functions and virtual tables (vtables). Here's a pretty neat system that I invented (based on C++) for a programming exercise:
(source: goblin.tkk.fi)
The constructors allocate memory and then call the class' init function where the memory is initialized. Each init function should also contain a static vtable struct that contains the virtual function pointers (NULL for pure virtual). Derived class init functions call the superclass init function before doing anything else.
A very nice API can be created by implementing the virtual function wrappers (not to be confused with the functions pointed to by the vtables) as follows (add static inline in front of it, if you do this in the header):
int playerGuess(Player* this) { return this->vtable->guess(this); }
Single inheritance can be done by abusing the binary layout of a struct:
(source: goblin.tkk.fi)
Notice that multiple inheritance is messier as then you often need to adjust the pointer value when casting between types of the hierarchy.
Other type-specific data can be added to the virtual tables as well. Examples include runtime type info (e.g. type name as a string), linking to superclass vtable and the destructor chain. You probably want virtual destructors where derived class destructor demotes the object to its super class and then recursively calls the destructor of that and so on, until the base class destructor is reached and that finally frees the struct.
There is the GObject library:
The GLib Object System, or GObject, is a free software library (covered by the LGPL) that provides a portable object system and transparent cross-language interoperability. GObject is designed for use both directly in C programs and through bindings to other languages.
The traditional solution is the function pointer struct. I emphasize traditional. I can tell you what sort of code I wrote in PL/I and C years ago, but I don't claim to speak for the state of the 'art' if you can call this art.
There are many variations on this, and the below is a bit of a compromise.
struct SortOfAnAbstractClass {
int (*function1)(SortOfAnAbstractClass* this, int arg1, int arg2, char * arg3);
void (*function2)(SortOfAnAbstractClass* this, char *arg);
};
struct SortOfDerived {
struct SortOfAnAbstractClass base;
int instanceVariable1;
};
SortOfAnAbstractClass getMeOne() {
SortOfDerived *d = malloc(sizeof SortOfDerived);
memset(d, 0, sizeof SortOfDerived);
d->function1 = myf1;
d->function2 = myf2;
return &d->base;
};
and then 'myf1' and 'myf2' cast their 'this' parameters and go to town. You can extend this to look ever more like a full virtual dispatch.
Another common variation from the mists of time:
struct SortOfAClass {
void *creatorInfo;
int (*function1)(SortOfAnAbstractClass* this, int arg1, int arg2, char * arg3);
void (*function2)(SortOfAnAbstractClass* this, char *arg);
};
In this variation, there's no inheritance by inclusion. The derived classes each put their private state into their own object in creatorInfo.
Take a look at the GObject library: http://library.gnome.org/devel/gobject/2.22/.
There are many variations to doing OO programming in C. The way I prefer to do it is to define one class per a header file. You'll notice a constructor new_testclass() which just initializes your function pointers and returns a pointer to an allocated class/struct. Also any function takes pointer to the class in the first parameter (something c++ does, but conceals).
testclass.h
#ifndef MALLOC_H
#include<malloc.h>
#endif
struct _testclass
{
int a;
int b;
int (*sum)(struct _testclass *obj);
};
typedef struct _testclass testclass;
int f_sum (testclass *obj)
{
return obj->a + obj->b;
}
testclass* new_testclass()
{
testclass *temp;
temp = (testclass*)malloc(sizeof(testclass));
temp->sum = &f_sum;
return temp;
}
Then you can simply use it.
testclass.c
#include <stdio.h>
#include "testclass.h"
int _tmain(int argc, _TCHAR* argv[])
{
int result;
testclass *testclass1;
testclass1 = new_testclass();
testclass1->a = 5;
testclass1->b = 8;
result = testclass1->sum(testclass1);
printf("%d\n",result);
free(testclass1);
return 0;
}
Of course there are several important aspects of object oriented programming missing here, but this provides a simple method for basic abstraction. I'd imagine inheritance would require some kind of funky preprocessor trick if it could be done at all.