Re: [Boost-users] passing comma to macro

From: Peter Bartlett (pete_at_[hidden])
Date: 2008-03-13 10:57:29

Quoting "Warlich, Christof \(Christof\)" <cwarlich_at_[hidden]>:

> Hi,
>
> the following code does not compile:
>
> #include <boost/preprocessor/if.hpp>
> #include <boost/preprocessor/comma.hpp>
> #include <boost/type_traits/is_convertible.hpp>
> #include <iostream>
> class A {};
> int main(void) {
>     BOOST_PP_IIF(boost::is_convertible<A BOOST_PP_COMMA() A>::value, //
> condition
>                  std::cout << "ifbranch";,                           //
> then
>                  std::cout << "elsebranch";);                        //
> else
>     return 0;
> }

There's a few things wrong with this.
Firstly you write BOOST_PP_IIF but include <boost/preprocessor/if.hpp>  
- you probably meant BOOST_PP_IF.

But more seriously your "condition" is not a number known at  
preprocessor-time - i.e. it won't evaluate to a number at  
pre-processor time, only at compile-time when the  
boost::is_convertible template magic kicks in. Thus BOOST_PP_IF  
wouldn't expand as you expect.

However, you aren't even seeing an error related to that problem.

BOOST_PP_COMMA() is indeed being evaluated before the BOOST_PP_IIF()  
and thus the compiler is complaining about 4 arguments to the latter  
macro - compilers do vary in how they expand macros - search the  
developer's list for Paul Mensonides' essay "How macro expansion  
works" for more - but you will be safe if your _force_ the COMMA macro  
to be expanded after the IF one. The following trick occurs over and  
over in writing preprocessor code:

BOOST_PP_IF( X , BOOST_PP_COMMA , BOOST_PP_EMPTY )()

Because BOOST_PP_COMMA and BOOST_PP_EMPTY are function-like macros  
they cannot possibly expand until they are "next to" the (). This can  
only happen once the BOOST_PP_IF has been evaluated. I.e. the above  
example will evaluate to a comma if X is non-zero and nothing  
otherwise....

I'd like to know if there is common name for this trick as I find  
'"macro name inside the IF, macro arguments outside"-trick' a bit  
verbose when discussing with  colleagues.

Pete