determine type of void

[rotis23]

Hi All,

I have a void* that is a pointer to either a char* or an int.

How do I determine which type the pointer is pointing to?

Thanks for your help,

rotis23

[Salem]

You don't

[itsme86]

You can't directly from the pointer. You have to use some form of a trick.

Usually, a second parameter is passed to a function receiving type void * that specifies the type. Something like:

void func(void *ptr, int type)
{
  if(type == 1)
  {
    // do char * stuff
  }
  else
  {
    // do int stuff
  }
}

Another option would be to embed the type somewhere in the pointer like at the head of it.

void func(void *ptr)
{
  int type = *(int *)ptr;

  ((int *)ptr)++;

  if(type == 1)
  // etc., same as code above.
}

[anonytmouse]

You can use a structure/union to pass around multiple types neatly:

enum var_type { TYPE_INT, TYPE_CHARPTR };

struct variant_t
{
    enum var_type;

    union 
    {
         int int_val;
         char* charptr_val;
    } var_value;
};

For a mega sample see the Windows VARIANT type.

[itsme86]

Here's an example of usage:

itsme@dreams:~/C$ cat void.c
#include <stdio.h>

enum { STR_TYPE, INT_TYPE };

void func(void *ptr, int type)
{
  switch(type)
  {
    case STR_TYPE:
      printf("It's a str! The str is: %s\n", (char *)ptr);
      break;
    case INT_TYPE:
      printf("It's an int! The int is: %d\n", *(int *)ptr);
      break;
    default:
      printf("It's an unknown type!\n");
      break;
  }
}

int main(void)
{
  char str[] = "Hello, world!";
  int num = 86;

  func(str, STR_TYPE);
  func(&num, INT_TYPE);

  return 0;
}

And when I run it...

itsme@dreams:~/C$ ./void
It's a str! The str is: Hello, world!
It's an int! The int is: 86
itsme@dreams:~/C$

[rotis23]

Thanks people!