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

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

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

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

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

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

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