Methods in C

Published by Paul Buetow 2016-11-20

You can do some sort of object-oriented programming in the C Programming Language. However, that is very limited. But also very easy and straightforward to use.


Let's have a look at the following sample program. All you have to do is to add a function pointer such as "calculate" to the definition of struct "something_s". Later, during the struct initialization, assign a function address to that function pointer:

#include <stdio.h>

typedef struct {
    double (*calculate)(const double, const double);
    char *name;
} something_s;

double multiplication(const double a, const double b) {
    return a * b;

double division(const double a, const double b) {
    return a / b;

int main(void) {
    something_s mult = (something_s) {
        .calculate = multiplication,
        .name = "Multiplication"

    something_s div = (something_s) {
        .calculate = division,
        .name = "Division"

    const double a = 3, b = 2;

    printf("%s(%f, %f) => %f\n",, a, b, mult.calculate(a,b));
    printf("%s(%f, %f) => %f\n",, a, b, div.calculate(a,b));

As you can see, you can call the function (pointed by the function pointer) the same way as in C++ or Java via:

printf("%s(%f, %f) => %f\n",, a, b, mult.calculate(a,b));
printf("%s(%f, %f) => %f\n",, a, b, div.calculate(a,b));

However, that's just syntactic sugar for:

printf("%s(%f, %f) => %f\n",, a, b, (*mult.calculate)(a,b));
printf("%s(%f, %f) => %f\n",, a, b, (*div.calculate)(a,b));


pbuetow ~/git/blog/source [38268]% gcc methods-in-c.c -o methods-in-c
pbuetow ~/git/blog/source [38269]% ./methods-in-c
Multiplication(3.000000, 2.000000) => 6.000000
Division(3.000000, 2.000000) => 1.500000

Not complicated at all, but nice to know and helps to make the code easier to read!

The flaw

However, that's not really how it works in object-oriented languages such as Java and C++. The method call in this example is not a method call as "mult" and "div" in this example are not "message receivers". I mean that the functions can not access the state of the "mult" and "div" struct objects. In C, you would need to do something like this instead if you wanted to access the state of "mult" from within the calculate function, you would have to pass it as an argument:


How to overcome this? You need to take it further.

Taking it further

If you want to take it further, type "Object-Oriented Programming with ANSI-C" into your favourite internet search engine, you will find some crazy stuff. Some go as far as writing a C preprocessor in AWK, which takes some object-oriented pseudo-C and transforms it to plain C so that the C compiler can compile it to machine code. This is similar to how the C++ language had its origins.

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