union versus void pointer

Question

What would be the differences between using simply a void* as opposed to a union? Example:

struct my_struct {
    short datatype;
    void *data;
}

struct my_struct {
    short datatype;
    union {
        char* c;
        int* i;
        long* l;
    };
};

Both of those can be used to accomplish the exact same thing, is it better to use the union or the void* though?

Answer 1

It really depends on the problem you're trying to solve. Without that context it's really impossible to evaluate which would be better.

For example, if you're trying to build a generic container like a list or a queue that can handle arbitrary data types, then the void pointer approach is preferable. OTOH, if you're limiting yourself to a small set of primitive data types, then the union approach can save you some time and effort.

Answer 2

I had exactly the case in our library. We had a generic string mapping module that could use different sizes for the index, 8, 16 or 32 bit (for historic reasons). So the code was full of code like that:

if(map->idxSiz == 1) 
   return ((BYTE *)map->idx)[Pos] = ...whatever
else
   if(map->idxSiz == 2) 
     return ((WORD *)map->idx)[Pos] = ...whatever
   else
     return ((LONG *)map->idx)[Pos] = ...whatever

There were 100 of lines like that. In a first step I changed it an union and I found it was more readable.

switch(map->idxSiz) {
  case 1: return map->idx.u8[Pos] = ...whatever
  case 2: return map->idx.u16[Pos] = ...whatever
  case 3: return map->idx.u32[Pos] = ...whatever
}

This alowed me to see better what was going on and I could then decide to remove completely the idxSiz variants using only 32 bit indexes. But this was only possible once the code got more readable. PS: That was only a minor part of our project which is about several 100 thousands of line of code written by people who do not exist anymore. So the changes in code are gradual so as not to break the applications.

Conclusion: Even if people are less used to the union variant, I prefer it because it can make the code much lighter to read. On big projects, it is extremely important to make the code more readable, even if it is yourself that will read it later.

Edit: Added the comment, as comments do not format code:

The change to switch came before (this is now the real code as it was)

switch(this->IdxSiz) { 
  case 2: ((uint16_t*)this->iSort)[Pos-1] = (uint16_t)this->header.nUz; break; 
  case 4: ((uint32_t*)this->iSort)[Pos-1] = this->header.nUz; break; 
}

was changed to

switch(this->IdxSiz) { 
  case 2: this->iSort.u16[Pos-1] = this->header.nUz; break; 
  case 4: this->iSort.u32[Pos-1] = this->header.nUz; break; 
}

I shouldn't have combined all the beautification I did in the code and only show that step. But I posted my answer from home where I had no access to the code

Answer 3

If you build your code with -fstrict-aliasing (gcc) or similar options on other compilers, then you have to be very careful with how you do your casting. You can cast a pointer as much as you want, but when you dereference it, the pointer type that you use for the dereference must match the original type (with some exceptions). You can't for example do something like:

void foo(void * p)
{
   short * pSubSetOfInt = (short *)p ;
   *pSubSetOfInt = 0xFFFF ;
}

void goo()
{
   int intValue = 0 ;

   foo( &intValue ) ;

   printf( "0x%X\n", intValue ) ;
}

Don't be suprised if this prints 0 (say) instead of 0xFFFF or 0xFFFF0000 as you may expect when building with optimization. One way to make this code work is to do the same thing using a union, and the code will probably be easier to understand too.

Answer 4

The union reservs enough space for the largest member, they don't have to be same, as void* has a fixed size, whereas the union can be used for arbitrary size.

#include <stdio.h>
#include <stdlib.h>

struct m1 {
   union {
    char c[100];
   };
};

struct m2 {
    void * c;
 };


 int
 main()
 {
printf("sizeof m1 is %d ",sizeof(struct m1));
printf("sizeof m2 is %d",sizeof(struct m2));
exit(EXIT_SUCCESS);
 }

Output: sizeof m1 is 100 sizeof m2 is 4

EDIT: assuming you only use pointers of the same size as void* , I think the union is better, as you will gain a bit of error detection when trying to set .c with an integer pointer, etc'. void* , unless you're creating you're own allocator, is definitely quick and dirty, for better or for worse.

Answer 5

It's more common to use a union to hold actual objects rather than pointers.

I think most C developers that I respect would not bother to union different pointers together; if a general-purpose pointer is needed, just using void * certainly is "the C way". The language sacrifices a lot of safety in order to allow you to deliberately alias the types of things; considering what we have paid for this feature we might as well use it when it simplifies the code. That's why the escapes from strict typing have always been there.