cyclic dependency - structures and function pointer referencing each other

Question

I am not able to compile the following program because of cyclic dependency between a structure and a function pointer.

// fnPtr.cpp : Defines the entry point for the console application. //

#include "stdafx.h"
#include "stdlib.h"
//typedef gNewType;
typedef void (*ptr)(gStruct *pStr, int pNm);
struct gInStruct{
char *sAlpBet;
ptr sFunPtr;    
};

struct gStruct{
//gNewType sNtype;
gInStruct sInStruct;
int sNum;
};
//typedef void (*ptr)(gStruct *pStr, int pNm);
void myFun(char *pStr1, int pNum){

return ;
}
int _tmain(int argc, _TCHAR* argv[])
{
void (*fnPtr)(char *pStr, int pNm);
char lAlphabet = 'a';
fnPtr = myFun;
gStruct *lStructPtr = (gStruct*)malloc(sizeof(gStruct));
lStructPtr->sNum = 10;
lStructPtr->sInStruct.sAlpBet = &lAlphabet;
lStructPtr->sInStruct.sFunPtr = fnPtr;
return 0;
}

I am getting following errors:

Error 7 error C2039: 'sFunPtr' : is not a member of 'gInStruct' c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 31

Error 3 error C2062: type 'int' unexpected c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 7

Error 1 error C2065: 'gStruct' : undeclared identifier c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 7

Error 2 error C2065: 'pStr' : undeclared identifier c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 7

Error 4 error C2146: syntax error : missing ';' before identifier 'sFunPtr' c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

Error 5 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

Error 6 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

If I comment first typedef and uncomment second typedef

Error 4 error C2039: 'sFunPtr' : is not a member of 'gInStruct' c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 31

Error 1 error C2146: syntax error : missing ';' before identifier 'sFunPtr' c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

Error 2 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

Error 3 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\users\saurabh17g\documents\visual studio 2010\projects\fnptr\fnptr\fnptr.cpp 10

Please let me know a possible answer. Thanks in advance, Saurabh

Answer 1

You should declare a struct before you can use it.

Just add struct gStruct; before the typedef, and it will be fine.

Answer 2

// fnPtr.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "stdlib.h"
//typedef gNewType;
struct gStruct;
typedef void (*ptr)(gStruct *pStr, int pNm);
struct gInStruct{
char *sAlpBet;
ptr sFunPtr;    
};

struct gStruct{
//gNewType sNtype;
gInStruct sInStruct;
int sNum;
};
//typedef void (*ptr)(gStruct *pStr, int pNm);
void myFun(gStruct *pStr1, int pNum){

return ;
}
int _tmain(int argc, _TCHAR* argv[])
{
void (*fnPtr)(gStruct *pStr, int pNm);
char lAlphabet = 'a';
fnPtr = myFun;
gStruct *lStructPtr = (gStruct*)malloc(sizeof(gStruct));
lStructPtr->sNum = 10;
lStructPtr->sInStruct.sAlpBet = &lAlphabet;
lStructPtr->sInStruct.sFunPtr = fnPtr;
return 0;
}