Quoting "Warlich, Christof \(Christof\)" <cwarlich@alcatel-lucent.com>:
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