Re: Prohibit use of break/continue in a statement

[David Brown <david.brown@hesbynett.no>]

On 31/01/2022 16:54, Hirrolot via Gcc-help wrote:
> I have a macro that expands to a for-loop. It is used as follows:
> 
>     MACRO(...) {
>         // User code
>     }
> 
> The for-loop is used to open a new scope with a new variable; thus,
> `MACRO(...) { ... }` would be a proper C statement. The loop itself is
> executed only once.
> 
> The problem is that if a user uses `break` or `continue` like this:
> 
>     while (i < 10) {
>         MACRO(...) {
>            break;
>             // User code...
>        }
>     }
> 
> Then that `break` will apply to the for-loop generated by `MACRO`, not
> to the outer while-loop. The same holds for `continue`. This is
> unexpected behaviour, and I would like to prohibit the use of the
> `break`/`continue` statements in a user statement placed after
> `MACRO`. Ideally, this should somehow trigger a compilation
> error/warning.
> 
> It should be clarified that `MACRO` is not of a loop itself: it is not
> a for-each macro or something like this; using that for-loop is just
> an implementation detail. I would be also happy with getting rid of
> that for-loop but I have no idea how. If I just generate a variable
> like this:
> 
>     #define MACRO(...) int x; /* Some other stuff */
> 
> Then `MACRO(...) { ... }` will no longer be a single C statement.
> Moreover, two occurrences of `MACRO` calls in a single scope will
> result in a compilation error, which is also quite unfortunate.
> 
> Is it possible to trigger a compilation warning/error for
> `break`/`continue` with GCC?
> 

The only thing I can think of is:

#define break DON'T USE BREAK HERE
#define continue DON'T USE CONTINUE HERE

That should cause a compilation error if "break" or "continue" are used.
 Of course, you need to use the #define and a matching #undef in the
places MACRO is used.  And it's undefined behaviour to use keywords as
macro names, so other bad things might happen.

A far better idea would be to explain what you are trying to achieve
with your MACRO, and show your current definition.  Then people could
perhaps give you useful advice!

David