Written by Paul Buetow 2016-11-20

Methods in C

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

Example

Lets have a look at the following sample program. Basically 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", mult.name, a, b, mult.calculate(a,b));
    printf("%s(%f, %f) => %f\n", div.name, 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", mult.name, a, b, mult.calculate(a,b));
printf("%s(%f, %f) => %f\n", div.name, a, b, div.calculate(a,b));

However, that's just syntactic sugar for:

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

Output:

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!

Taking it further

If you want to take it a lot 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 actually similar to how the C++ language had its origins.

E-Mail me your throughts at comments@mx.buetow.org!