Shared pointers???

[gedumer]

Hi,

in C... is there any such thing as an "any" type pointer? In the example below, is there any way that I could define function "x" to accept a pointer of type "struct a" or "struct b", or any other structure I might create with "int i", so I wouldn't have to create 2 separate functions that do exactly the same thing? Let's assume I have a valid reason for having 2 separate structures. I would just like to share function "x" and eliminate function "y". I could have infinite structures that have "int i" and I'd like to use generic function "x" for all of them. Is there a way?

#include <stdio.h>
// Structure "a"
typedef struct
{
  int i;
} a;
// Structure "b"
typedef struct
{
  int i;
  char s[5];
} b;
// Function "x"
void x(a *p)
{
  (*p).i = 10;
}
// Function "y"
void y(b *p)
{
  (*p).i = 20;
}
int main(void)
{
  a a1;
  b b1;
  x(&a1);
  y(&b1);
  printf("a1.i = %d,  b1.i = %d\n", a1.i, b1.i);
  return 0;
}

[Stefan Pendl]

The following might work:

Code: [Select]

// Function "x"
void x(void *p)
{
  (*p).i = 10;
}
// Function "y"
void y(void *p)
{
  (*p).i = 20;
}


[gedumer]

It appears to be an unsolvable problem, from a practical standpoint at least, so I've decided to go in another direction. Thanks.

[pgoh]

The following might work:

Code: [Select]

// Function "x"
void x(void *p)
{
  (*p).i = 10;
}
// Function "y"
void y(void *p)
{
  (*p).i = 20;
}


The code above will not work. That's because there is no type info contained within a void pointer. A better option would be to cast the point to an int pointer, and then set the value of the int pointer to whatever new value you wish.

Here's some code that does just that.

Code: [Select]

#include <stdio.h>
// Structure "a"
typedef struct {
int i;
} a;

// Structure "b"
typedef struct {
int i;
char s[5];
} b;

// Function "x"
void x(a * p)
{
(*p).i = 10;
}
// Function "y"
void y(b * p)
{
(*p).i = 20;
}

void z(void *p) {
int *i = (int *)p;
*i = 55;
}

int main(void)
{
a a1;
b b1;
x(&a1);
y(&b1);
printf("a1.i = %d,  b1.i = %d\n", a1.i, b1.i);

z(&a1);
z(&b1);
printf("a1.i = %d,  b1.i = %d\n", a1.i, b1.i);

return 0;
}

A couple of caveats you need to be aware of when using this method:

• There is no type safety. Function z assumes that whatever is passed in can be cast to an integer and then have it's value set
• Function z requires that anything that is passed in, the first struct field is an int
• For more complex data, you might want to make a struct the first field in structs a and b

Of course, what you're attempting to do sounds a lot like polymorphism. If that's the case, you might be better off using C++ instead of C.