gcc C language forward declaration problem

yousafsajjad · 07-21-2010, 02:34 PM

this is strange .. i did use forward declaration in c++ but in c it is giving me an error ... for example:

Code:

#include <stdio.h>
#include <string.h>

typedef struct Student S;

typedef struct Student{
char name[100];
}S;


int main()
{
char c[100];
S s;
strcpy(s.name,"yousaf");
printf("%s\n",s.name);
return 1;
}

if i compile the above code using:

Code:

gcc test.c -o test

it give me an error of redefinition
but if i use

Code:

 g++ -x c test.c -o test

it works just fine. does anyone know whats the trick around this problem

johnsfine · 07-21-2010, 03:24 PM

Quote:

Originally Posted by yousafsajjad

this is strange

It was a surprise to me too.

I didn't know that C++ (g++ anyway) lets you repeat a typedef within one compilation unit.

You can't repeat most other definitions within a compilation unit even if the two definitions are identical.

Strip away all the irrelevant complexity of your example. The following code works in c++ and fails in c

Code:

typedef int Int;
typedef int Int;

Quote:

i did use forward declaration in c++ but in c it is giving me an error

You are mixing a typedef with a forward declaration. The typedef itself is not a forward declaration.

Code:

typedef struct Student S;

In both C and C++ that is a forward declaration of struct Student combined with a typedef of S.

It is not a forward declaration of S.

Code:

typedef struct Student{
char name[100];
}S;

That is both a definition of struct Student and a redundant second typedef of S.

I don't know why that is correct in C++.

In either C or C++, the extra typedef serves no useful purpose. I think the code you want is

Code:

typedef struct Student S;
struct Student{
char name[100];
};

That should correctly define S in either C or C++.

For any lurkers who wonder why someone would do any of that in the first place (vs. combine the typedef and struct declaration without any forward declaration), typically the programmer would be working toward something like

Code:

typedef struct Student S;

struct Student{
   char name[100];
   S* next;
};

So (in C) S must be declared before Student is defined.

yousafsajjad · 07-21-2010, 04:14 PM

hmm let me try

yousafsajjad · 07-21-2010, 04:29 PM

[QUOTE=johnsfine;4041091]

Code:

typedef struct Student S;
typedef struct Student{
S *ap;
char name[100];
};

this sure compiles fine but it gives out a warning "warning: useless storage class specifier in empty declaration". Do you know how to fix that?

paulsm4 · 07-21-2010, 05:58 PM

Here's a slightly more useful version, without any "forward declarations":

1. There's a "typedef" to alias each struct definition
2. One struct depends on another
3. This example compiles/runs without any errors or warnings:

Code:

/* EXAMPLE 1 */
#include <stdio.h>
#include <string.h>

typedef struct ClassRec
{
  char name[100];
  int  grade;
}
  C;

typedef struct StudentRec
{
  char name[100];
  C class;
}
  S;

int
main(int argc, char *argv[])
{
  S s;
  strcpy(s.name, "yousaf");
  strcpy(s.class.name, "math");
  s.class.grade = 100;
  printf("%s\n", s.name);
  return 0;
}

This, I believe, is basically what you're trying to accomplish:

Code:

/* EXAMPLE 2 */
#include <stdio.h>
#include <string.h>

typedef struct ClassRec C;

typedef struct StudentRec
{
  char name[100];
  C class;
}
  S;

struct ClassRec
{
  char name[100];
  int  grade;
};

int
main(int argc, char *argv[])
{
  S s;
  ...
  <= FAILS WITH COMPILE ERROR:
x.c:9: error: field `class' has incomplete type

I think you're probably best sticking with "EXAMPLE 1": I don't think you'll be able to simplify much further.

IMHO .. PSM

PS:
This is perverse, but it might be something close to what you're looking for:

Code:

/* EXAMPLE 3 */
#include <stdio.h>
#include <string.h>

typedef struct ClassRec *ClassRecPtr;

typedef struct StudentRec
{
  char name[100];
  ClassRecPtr c;
}
  S;

typedef struct ClassRec
{
  char name[100];
  int  grade;
}
  C;

int
main(int argc, char *argv[])
{
  S s;
  C c;
  s.c = &c;
  strcpy(s.name, "yousaf");
  strcpy(s.c->name, "math");
  s.c->grade = 100;
  printf("student: %s, class: %s, grade: %d\n",
    s.name, s.c->name, s.c->grade);
  return 0;
}

johnsfine · 07-21-2010, 08:13 PM

Quote:

Originally Posted by yousafsajjad

Code:

typedef struct Student S;
typedef struct Student{
S *ap;
char name[100];
};

this sure compiles fine but it gives out a warning "warning: useless storage class specifier in empty declaration". Do you know how to fix that?

Notice the typedef keyword you have that doesn't define anything (notice it wasn't there in the version I suggested). That is what the warning means.

paulsm4 · 07-21-2010, 09:42 PM

Also not that it will NOT work if you try to "typedef the forward reference" to anything OTHER than a pointer.

That's the gist of what I was trying to show in my examples.

yousafsajjad · 07-22-2010, 04:09 PM

That really clears up the whole thing .. thank you all