From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2006-01-29 10:06:57
> -----Original Message-----
> From: boost-bounces_at_[hidden]
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Tobias Schwinger
> >> [...] which creates pathological input that IF cannot deal with:
> >> [...]
> While we're at it:
> This code (taken from boost/typeof/vector.hpp, line 106)
> BOOST_PP_IF(n, BOOST_PP_EMPTY(), class T = void)
> seems to contain a similar (but more subtle) problem.
> Emptiness (well, deferred emptiness in this case, to be
> precise) is passed to BOOST_PP_IF as the second argument, and
> empty input (to the macros used by BOOST_PP_IF internally)
> results in undefined behaviour -- so it's up to the
> preprocessor whether things work correctly or not.
This is exactly right. In current C++, empty arguments amount to undefined
behavior. Many preprocessors allow it, and C99 requires it to work, but it
*shouldn't* work in current C++.
> For a quick fix, a more bullet-proof version could look like this
> BOOST_PP_IF(n,BOOST_PP_EMPTY,BOOST_PP_IDENTITY(class T = void))()
> BOOST_PP_EXPR_IIF(BOOST_PP_NOT(n), class T = void)
The latter of the two is the better solution, IMO. You could also do:
)(1)(class T = void)
This allows the 'class T = void' part to be a macro invocation that is lazily
evaluated (or at least, is _supposed_ to be lazily evaluated). E.g.
)(1)( EXPENSIVE_MACRO_INVOCATION() )
...where EXPENSIVE_MACRO_INVOCATION() is only invoked if 'n' is 0.
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