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- gt chars       I have a function initializeString   that returns a pointer to a PString   PString   initializeString         PString  str      str- gt length    amp length      return str      It is clear that I am doing something very wrong with my pointers here  because the str- gt length    amp 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

User · Answer

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));

User · Answer

Allocate memory to hold chars    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   typedef struct PString           char  chars          int   length  PString  self     PString   int length PString  self        return strlen self- gt chars      PString  initializeString int n        PString  str   malloc sizeof PString         str- gt chars   malloc sizeof char    n       str- gt length   length       str- gt 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- gt chars   Hello        printf   n d   p- gt length p        return 0

User · Answer

The pointer str is never allocated  It should be malloc d before use

User · Answer

You can use also  void    void pointer  to send an address to the function   typedef struct pstring t       char   chars      int  length  void      PString   int length void  self        return strlen   PString  self - gt chars      PString initializeString         PString str      str length    amp length      return str     int main         PString p   initializeString         p chars    Hello        printf  Length   i n   p length  amp p         return 0      Output   Length  5

User · Answer

Maybe I am missing something here  but did you allocate any memory for that PString before you accessed it   PString   initializeString         PString  str      str    PString    malloc sizeof PString        str- gt length    amp length      return str

[c] Function pointer as a member of a C struct

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