Question

I am trying to explore OOP in C. I am however a C n00b and would like to pick the brilliant brains of stackoverflow 🙂

My code is below:

#include <stdio.h>
#include <stdlib.h>

typedef struct speaker {
    void (*say)(char *msg);
} speaker;

void say(char *dest) {
  printf("%s",dest);
}

speaker* NewSpeaker() {
  speaker *s;
  s->say = say;
  return s;
}

int main() {
  speaker *s = NewSpeaker();
  s->say("works");
}

However I’m getting a segfault from this, if I however remove all args from say and make it void, I can get it to work properly. What is wrong with my current code?

Also. While this implements a form of object in C, I’m trying to further implement it with inheritance, and even overriding/overloading of methods. How do you think I can implement such?

Thank You!

Answer 1

In your code, NewSpeaker() doesn’t actually create a “new” speaker. You need to use a memory allocation function such as malloc or calloc.

speaker* NewSpeaker() {
   speaker *s = malloc(sizeof(speaker));
   s->say = say;
   return s;
}

Without assigning the value from, for example, the return value of malloc, s is initialized to junk on the stack, hence the segfault.