Macros

[Schwarzhelm]

I have a question about using macros in C.
First, I am trying to divide and return both the answer and the remainder from a macro. Right now I have:

#define DIVMOD(x,y,d,r) ((y)==0 ? d=0; r=0 : (d = (x)/(y); r = (x) % (y))

which I know is not correct. Any suggestions on how I could do this?

Thank you.

[Shiro]

Why would you use a macro for that? You can't return values from macros, a macro is not a function, it is a substitution. You could try to create a macro which has two "output-variables". In the "call" to the macro pass the addresses of those variables and in the macro use the usual operations you would do to assign values to such variables.

[DavT]

This works for me:

#define DIVMOD(x,y,d,r) d = ((y)==0) ? 0, r=0 : (x)/(y), r = (x) % (y)

but I agree with Shiro that this is quite a nasty bit of code. A function would be much neater...

void DivMod(int x, int y, int * pd, int * pr)
{
  if (y == 0)
    *pd = *pr = 0;
  else
  {
    *pd = x / y;
    *pr = x % y;
  }
}

...and unless you are doing loads of these operations isn't going to significantly slow down your program.

If you insist on using a macro use something like:

#define DIVMOD(x,y,pd,pr) *pd = ((y)==0) ? 0, *pr=0 : (x)/(y), *pr = (x) % (y)

where pd and pr are pointers.

[quzah]

Originally posted by DavT
If you insist on using a macro use something like:

#define DIVMOD(x,y,pd,pr) *pd = ((y)==0) ? 0, *pr=0 : (x)/(y), *pr = (x) % (y)

where pd and pr are pointers.

What's the purpose of using pointers? It's just a macro. It's not a function call.

Quzah.

[DavT]

It's probably just a personal hang-up, but I I really dislike macros.

I think they lead to confusing code. The only excuse I've ever come across for using a macro where a function would do the job is in some kind of real-time signal processing situation where you can't afford the overhead of a function call and the compiler doesn't support the 'inline' (or similar) keyword.

In the flow of the program I find it really disconcerting (or downright confusing) to find something that looks like a function call but (taking an example at random) returns values via non-pointer arguments.

Just my personal point of view....