Function-like macro

[Richard Bos]

Re: Function-like macro

Dale Hagglund <dale.hagglund@ gmail.com> wrote:
[color=blue]
> Others have already pointed out that you're probably much better off
> using an inline function. It's more easily readable and editable.
> That said, GCC does have a non-standard extension that allows you to
> do what you want:
>
> #define A_FROM_B(b) \
> ({ \
> int __b = (b); \
> int __a; \
> if (__b < 10) \
> __a = __b; \
> else \
> __a = 2*__b; \
> __a; \
> })[/color]
[color=blue]
> * I introduced the __b variable to prevent multiple evaluation
> of b. (Functions get you this for free, of course.)[/color]

Unfortunately it also makes the macro non-portable, since all
identifiers starting with __ (or _ plus capital letter) are reserved for
the implementation. __b is simple enough that it could clash with
something defined in a system header.
[color=blue]
> * As I mentioned, as far as I know this is non-standard.[/color]

'tis.
[color=blue]
> * Because macros are based on textual substitution, it's a
> good idea to introduce local variable names that aren't
> likely to collide with names used inside the macro
> arguments. (Again, functions completely avoid this issue.)[/color]

But make sure they're yours to use.

Richard

[pete]

Re: Function-like macro

Richard Bos wrote:
[color=blue]
> #define A_FROM_B(b) ((b)<10? (b): 2*(b))
>
> Note the parens around b in the definition of the macro.
> They will save
> your bacon some day when you decide to call A_FROM_B(x+10).[/color]

Trivia point:
Only the first and last pair are needed for bacon saving.

#define A_FROM_B(b) ((b) < 10 ? b : 2 * (b))

? b :
means the same thing as
?(b):

--
pete

[Richard Bos]

Re: Function-like macro

pete <pfiland@mindsp ring.com> wrote:
[color=blue]
> Richard Bos wrote:
>[color=green]
> > #define A_FROM_B(b) ((b)<10? (b): 2*(b))
> >
> > Note the parens around b in the definition of the macro.
> > They will save
> > your bacon some day when you decide to call A_FROM_B(x+10).[/color]
>
> Trivia point:
> Only the first and last pair are needed for bacon saving.
>
> #define A_FROM_B(b) ((b) < 10 ? b : 2 * (b))
>
> ? b :
> means the same thing as
> ?(b):[/color]

In this case, perhaps. Howsoever, I'd advise getting into the habit of
parenthesisisin g anyway, for psychobabblical reasons.

Richard