Very basic OOP programming in C

**patishi** · 12-20-2013

Hi. I have just recentely learned on function pointers.
So now i know how to create a function and assign it a pointer.
So far so good.

But can someone give me a simple example on how I can create a struct and have a function that return a value that is inside that struct ?
And how can I create a constructor function in order to instantiate all the variables inside a struct?

just to explain myself better, let's say I have a struct person:

Code:

struct Person{
    int age;
    int(*getAge)(int);
};
typedef struct Person Person;

And a function that get's an int and returns it:

Code:

int returnInt(int x){
   return x;
}

now i can create a new Person:

Code:

Person p;
p.age = 30;

But how can i link the getAge pointer to the returnInt() function, so that it will be returning the age inside the Person ?
And how can I create some sort of constructor that will automatically initialize all the variables inside a Person struct?

(I can't do something like this:

Code:

struct Person{
   int age = 30;  // THIS WILL NOT COMPILE
........
};

Thx.

**c99tutorial** · 12-20-2013

Originally Posted by patishi

But how can i link the getAge pointer to the returnInt() function, so that it will be returning the age inside the Person ?
And how can I create some sort of constructor that will automatically initialize all the variables inside a Person struct?

You can't make construction automatic, but you can make a constructor function and make sure you call it. If the struct is opaque, then you can also be reasonably sure that the user will have to use your constructor (similar to fopen and fclose operate on FILE *).

If the object is not-opaque then of course there is no true information hiding, but for most purposes in C this is probably adequate. If not consider C++.

Person.h example

Code:

#pragma once
#define PERSON_STRMAX (1000)

typedef struct Person Person;
struct Person;

Person Person_construct();
Person *Person_print(Person *p);
Person *Person_set_name(Person *p, const char *name_str);

struct Person {
    int age;
    char name[PERSON_STRMAX];
    Person *(*print)(Person *);
    Person *(*set_name)(Person *, const char *name_str);
};

static inline Person
Person_default() {
    return (Person) {
        0,
        "",
        Person_print,
        Person_set_name
    };
}

Person.c example

Code:

#include "config.h"
#include "classes/Person.h"

Person
Person_construct()
{
    Person p = Person_default();
    return p;
}

Person *
Person_print(Person *p)
{
    printf("Name: %s\n"
           "Age: %d\n", p->name, p->age);
    return p;
}

Person *
Person_set_name(Person *p, const char *name_str)
{
    strncpy(p->name, name_str, PERSON_STRMAX);
    return p;
}

If you want to use it notice you have to pass a pointer to the object for each method call. Also, assigning function pointers is not strictly required. You could just use Person_set_name or Person_print and so on. In fact many C libraries like SDL and APR use this kind of convention.

Code:

Person p = Person_construct();
p.set_name(&p, "Bob");
p.age = 42;
p.print(&p);

**grumpy** · 12-20-2013

You can't.

To call a member function, it is necessary to have two items of information: the action to be performed (i.e. the body of the function) and a reference (or pointer) to the object being acted on. The two bits of information have to be available to the function body.

C++ and other OO languages achieve what you have in mind by passing additional information (the "this" pointer, "self" reference, or something similar - depending on language) when calling the function. In a compiled language, the compiler takes care of implicitly passing that information with every member function call, so the programmer doesn't have to. In an interpreted language, the interpreter takes care of that. Etc.

The thing is, in C, there is no implicit mechanism (i.e. one the that compiler does for you) for such things. If you want to achieve a similar effect, you have to fall back on explicitly passing that information. In other words, the *programmer* explicitly passes the required information every time a member function is called.

You can't have your cake and eat it to. In OO languages, it happens implicitly because the compiler (or interpreter, or whatever) takes care of the machinery. You can't expect it to happen by magic when using a language which doesn't require the compiler to support such machinery.

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