Tricking C: Function pointers in C structs

Software

So today in the l2l the question came up, "Can you use function pointers in C structs to get around the 'no functions in structs' thing." So like any good scientist I opened up vim and typed up a test:

#include <stdio.h>

int Foo_square(int a);

struct Foo {
int (*square)(int);
};

int main()
{
    struct Foo bar;
    bar.square = &Foo_square;
    printf("%dn", bar.square(4));

    return 0;
}

int Foo_square(int a)
{
    return a * a;
}

So, it works just fine. Now I decided to see about making it less messy like you would have with an object oriented language. I broke the file into a header file and source code file. *note* It has been brought up that I should have done the following header file in the more traditional .h is an interface with a .c file implementing the various functions. While it is tradition, I believe that it is extraneous for what I am trying to show here.

Header:

#ifndef FILE_H
#define FILE_H
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

typedef struct {
    void (*open_write)();
    void (*open_read)();
    void (*set_filename)(char*);
    FILE* fp;
    char file_name[100];
} File;

File* new_File();

File* this;

void File_open_write()
{
    this->fp = fopen(this->file_name,"w");
}

void File_open_read()
{
    this->fp = fopen(this->file_name,"r");
}

void File_set_filename(char* file_name)
{
    strncpy(this->file_name,file_name,99);
}

File* new_File()
{
    this = (File*)malloc(sizeof(File));
    this->open_write = &File_open_write;
    this->open_read = &File_open_read;
    this->set_filename = &File_set_filename;
    memset(this->file_name,(int)NULL,100);

    return this;
}
#endif

Source File:

#include "File.h"
#include <stdio.h>

int main()
{
    File* bar = new_File();
    bar->set_filename("test.txt");
    printf("%sn", bar->file_name);

    return 0;
}

The header file simply defines everything that will be needed for the File struct along with a fuction new_File() which creates a pointer for the struct, sets up the functions and returns the pointer. This makes the source file extremely clean, because all you do is call new_File() and everything is done. You can now manipulate the struct similarly to how you would manipulate an object. This actually looks a lot like how php classes work... god dammit...

Created: 2009-04-05 23:52:05