http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58887
Bug ID: 58887
Summary: Allow recursion in varadic macros?
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: preprocessor
Assignee: unassigned at gcc dot gnu.org
Reporter: mtewoodbury at gmail dot com
With good reason, recursion in macro definitions is suppressed -- it leads to
infinite expansion loops and subsequent software failure, HOWEVER
Recursion of varadic macros where the number of arguments in the argument list
decreases would necessarily stop expanding when the argument list became empty.
It would be nice if this could be handled easily, but doing it requires an
extension -- something beyond what the standard calls for/allows.
I can see two ways to do this -- there may be other ways as well:
1) Allow recursion as long as the new invocation has fewer arguments
than the current invocation.
This is the general solution, but should only be allowed if some
special mode is set, such as a command line switch or 'pragma'
2) Make the number of arguments easily available.
There is a way to write a varadic macro that return a count of
its arguments, but it has severe limitations -- it will necessarily
have an upper bound on the number of arguments it can count and
that limit will be half the implementation limit the preprocessor
puts on the number of arguments that can be passed to varadic
macros.
It would help if there were a predefined macro that did this
counting and did not have the upper bound limitation.
Once the argument count is available, it is possible to construct
the name of a macro for the correct number of arguments and have
those macros chain their expansion.
This solution is sub-optimal: macros for each argument count have
to be defined, which means there will be an arbitrary upper bound
on the number of arguments handled and puts a strain on preprocessor
resources.
For this reason, solution 1 is preferable.
