[boost] Rational Numbers - abs() again

From: jsiek_at_[hidden]
Date: 1999-12-14 19:11:09

Hi Paul,

I have still not heard of (or come up with myself) a
solution for this. As you've noticed, the obvious workarounds
don't work because VC++ is so broken. Perhaps we just
need to complain loudly to Microsoft. Does anyone have
connections with people on their compiler/library team?

Ciao,

Jeremy

Paul Moore writes:
 > I am still struggling with implementing abs() for rationals.
 > 
 > To summarise, I have
 > 
 >     template <typename IntType> class rational {
 >         ...
 >         friend rational abs (const rational &);
 >     }
 > 
 >     template <typename IntType>
 >     inline rational<IntType> abs (const rational<IntType> &r)
 >     {
 >         // point (1)
 >         return rational<IntType>(abs(r.num), r.den);
 >     }
 > 
 > Now, Koenig Lookup should find the friend abs when abs(r) is called with a
 > rational. But what about the *definition* of abs()? I use abs() here, as using
 > std::abs precludes IntType being a user-defined type. But to do this, I need to
 > add "using std::abs;" at point (1).
 > 
 > Now we have an issue - the using declaration needs to be protected by
 > BOOST_NO_STDC_NAMESPACE from <boost/config.hpp>.
 > 
 > But even with this, MSVC *still* has a problem - it fails to look for ::abs
 > [[ie, std::abs]] when compiling abs(rational). The error is
 > 
 > ..\..\boost/rational.hpp(248) : error C2784: 'class boost::rational<IntType>
 > __cdecl boost::abs (const class boost::rational<IntType> &)' : could not deduce
 > template argument for 'const class boost::rational<IntType> &' from 'const int'
 >         rational_example.cpp(61) : see reference to function template
 > instantiation 'class boost::rational<int> __cdecl boost::abs(const class
 > boost::rational<int> &)' being compiled
 > 
 > which seems to imply that MSVC isn't looking for the "normal" abs(int).
 > 
 > Can anybody suggest a workaround for this??? I ought to say that MSVC
 > compatibility is an absolute requirement - I use MSVC, so I'm not going to write
 > code which doesn't work with it (even allowing for the fact that I'm not sure
 > exactly what the standard would mandate on this issue!).
 > 
 > I'm thinking that I might be reduced to replacing abs(n) with (n < 0 ? -n : n).
 > This is OK, as we are requiring IntType to have "integer-like" semantics, but it
 > blows any hope of optimisation for a user-defuined IntType right out of the
 > water. (If someone can offer me a suitable implementation for
 > standard-conforming compilers, I'd be willing to put the hack in an ifdef
 > _MSC_VER).
 > 
 > All assistance gratefully accepted :-) I attach the header and test program.
 > 
 > Paul
 > 
 > 
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