Re: [boost] [preprocessor] Warning: Incoming

Subject: Re: [boost] [preprocessor] Warning: Incoming
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-06-27 17:25:15

On 06/27/2011 10:37 PM, Edward Diener wrote:

>> May I suggest the addition of a macro that strips parentheses but only
>> if they are present?
>
> I do not think this is possible. But I am out of the loop regarding
> pp-lib additions and Paul would really know if this could be done.

Code to do this was already given on the Boost ML by Steven Watanabe:
<http://article.gmane.org/gmane.comp.lib.boost.user/61011>

I've already been using this for a year in my code, and it works fine.

There is one little caveat with the above code, it's not strictly 
conforming to C99/C++0x so it doesn't work with Wave.
Replacing
(__VA_ARGS__, 2, 1)
by
(__VA_ARGS__, 2, 1, 0)

fixes the problem.

>
>> I have found this to be very useful for passing arguments that contain
>> commas easily.
>
> Can you give an example of what you are trying to do ?

Consider something like

BOOST_FOREACH(std::pair<A, B> a, mymap)

this fails because the preprocessor sees it as three arguments.

Assuming BOOST_FOREACH was using STRIP_PARENS on its arguments, you could do

BOOST_FOREACH((std::pair<A, B> a), mymap)

in the cases where it is necessary.

This is actually pretty much a requirement when you want to write macros 
that take template with multiple parameters as arguments.