Re: [boost] bug: cyclic header dependencies in type_traits

From: David Abrahams (dave_at_[hidden])
Date: 2006-04-06 10:45:59

"John Maddock" <john_at_[hidden]> writes:

>> This causes a problem for standards-compliant compilers because,
>> depending on where you enter the cycle, some template's declaration
>> will not be seen before it is used.  For example, entering at
>> is_scalar.hpp (which happens when you include optional.hpp) causes the
>> definition of is_class<> to be seen before the declaration of
>> is_scalar<>, but the former uses the later.
>>
>> The problem was introduced in the change from 1.32 to 1.33, when
>> is_abstract.hpp started being #included by is_convertible.hpp.
>>
>> For reasons I still don't fully understand, this doesn't always
>> actually cause a problem.  I think it has to do with the mysterious
>> BOOST_NO_IS_ABSTRACT macro; my understanding of the intricacies of
>> Boost configuration is very limited.  The place I ran into this was
>> using an EDG-based front end to parse the preprocessed output of Sun
>> CC 5.7 on solaris-sparc; input is "#include <boost/optional.hpp>";
>> using Boost 1.33.0 or 1.33.1.  I can't repro on linux or with gcc, nor
>> with the latest CVS version of Boost.
>>
>> But regardless, the cycle is still present in the latest CVS, and
>> cyclic header dependencies are usually a bug.
>
> This has come up before:  For any given specific compiler (the ones we know 
> about and support in Boost.Config) the dependency graph is not cyclic, but 
> is different from compiler to compiler.
>
> However, when using a front end about which we know nothing, it's possible 
> to have a configuration setup that fools the code into doing the wrong 
> thing.  Anyway, I'll change the config so that not setting 
> BOOST_NO_IS_ABSTRACT always takes a non-cyclic path.

I think it should be possible to arrange things so that even a naive
look at what-includes-what won't produce this sort of confusion, and
that would probably be worth it in the long run.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com