Function pointer as a member of a C struct

Question

I have a struct as follows, with a pointer to a function called "length" that will return the length of the chars member.

typedef struct pstring_t {     char * chars;     int (* length)(); } PString; 

I have a function to return the length of the characters from a pointer to a PString:

int length(PString * self) {     return strlen(self->chars); } 

I have a function initializeString() that returns a pointer to a PString:

PString * initializeString() {     PString *str;     str->length = &length;     return str; } 

It is clear that I am doing something very wrong with my pointers here, because the str->length = &length line causes an EXC_BAD_ACCESS signal in my debugger, as does `return strlen(self->chars). Does anyone have any insights into this problem?

I specifically want to be able have the initializeString() function return a pointer to a PString, and the length function to use a pointer to a PString as input. This is just an experiment in implementing a rudimentary object-oriented system in C, but I don't have a lot of experience dealing with pointers head-on. Thanks for any help you can give me.

Answer 1

Allocate memory to hold chars.

#include <stdio.h> #include <stdlib.h> #include <string.h>  typedef struct PString {         char *chars;         int (*length)(PString *self); } PString;  int length(PString *self) {     return strlen(self->chars); }  PString *initializeString(int n) {     PString *str = malloc(sizeof(PString));      str->chars = malloc(sizeof(char) * n);     str->length = length;      str->chars[0] = '\0'; //add a null terminator in case the string is used before any other initialization.      return str; }  int main() {     PString *p = initializeString(30);     strcpy(p->chars, "Hello");     printf("\n%d", p->length(p));     return 0; } 

Answer 2

My guess is that part of your problem is the parameter lists not matching.

int (* length)(); 

and

int length(PString * self) 

are not the same. It should be int (* length)(PString *);.

...woah, it's Jon!

Edit: and, as mentioned below, your struct pointer is never set to point to anything. The way you're doing it would only work if you were declaring a plain struct, not a pointer.

str = (PString *)malloc(sizeof(PString));