Re: Is it better to use a macro or a function?

Ben Pfaff wrote:
madhawi <madhawi.k@xxxxxxxxx> writes:

Is it better to use a macro or a function?

Some tasks can only be accomplished with a macro. Otherwise, use
a function: in general, they're safer.

An example:
I have in some of my programs a lot of pointers to structs.

struct a
{
char *s1;
char *s2
} *b;

char *s;

A general problem then is that such a pointer might be NULL and therefore referencing a member of that struct is illegal.

s=b->s1; /*if b is NULL this doesn't work well...*/

Now if what I want is something like this:
if (b)
s=b->s1;
else
s=NULL;

That is I want to assign *something* to s, if b is NULL (or for that matter b->s1 is NULL) I do want s to be NULL too. This could be written:
s=b?b->s1:NULL;

That's OK, so know if b isn't really a variable but rather a long complex expression I need to type that twice to get the desired effect.

#define spt(ptr)(!(ptr))?NULL:ptr

would make it possible to write
s=spt(b)->s1;

That trick is *not* possible with a function!
Oh, and before someone points it out, it is not entirely safe either, an expression in the place of b with side effects would not do what is intended! But it have helped me write some shorter and more (IMO) readable code. The expression b is evaluated twice if it is non-NULL, but as long as it doesn't have side effects and there is no speed requirements or the compiler is good enough at optimizing that doesn't matter.

It is even possible to chain them like this (obviously with some other declarations necessary)
res=spt(spt(spt(ptr)->member)->member)->member;

This allows for every step to be NULL and expands to (line breaks inserted for readability):
res=(!((!((!(ptr))?NULL:ptr->member))?NULL:(!(ptr))?NULL:
ptr->member->member))?NULL:(!((!(ptr))?NULL:ptr->member))?NULL:
(!(ptr))?NULL:ptr->member->member->member;

and doing exactly what I want but much more readable with the macro
than without. Of course typing
if (ptr)
{
if (ptr->member)
{
if (ptr->member->member)
res=ptr->member->member->member;
else
res=NULL;
}
else
res=NULL;
}
else
res=NULL;

would also produce the same result but would not (again IMO) be as readable.

Oh, once again, don't try this at home unless you understand the restriction "expression *must* be without side effects"
.

Relevant Pages

  • Re: dereferencing or not?
    ... Sorry, i'm using this struct: ... typedef struct database { ... char *hostname; ... i really want to use pointers in my code so i can force myself to start learning about them better but so far i only see drawbacks. ...
    (comp.unix.programmer)
  • Re: why the usage of gets() is dangerous.
    ... struct S { ... char c; ... clear at all where the buffer ends. ... making here, and I agreed already that bounded pointers, even while not ...
    (comp.lang.c)
  • Re: Copying Structures
    ... Because this emulator is intended for embedding there will be pointers to ... If a struct assignment does anything more than copying just the ... that does some sort of deep copy, ... source.s is a pointer to a char, but it happens to point to the first ...
    (comp.lang.c)
  • Re: structures, structures and more structures (questions about nested structures)
    ... typedef struct { ... char label; ... of all members, including pointers. ...
    (comp.lang.c)
  • [PATCH]a tar filesystem for 2.6.10-rc1-mm3
    ... +static int tarfs_readdir(struct file * filp, ... struct tarent *dir_tarent, *ent; ... +static int tarfs_readlink(struct dentry *dentry, char *buffer, int buflen) ...
    (Linux-Kernel)