[boost] Re: Rational Numbers - abs() again

From: jsiek_at_[hidden]
Date: 1999-12-14 19:14:52

Does this work if the type is complex? (perhaps this
doesn't apply here, but it does to many of my examples)

Thanks,

Jeremy

Boris Fomitchev writes:
 > Well,  abs:: already contains the conditional you need, just make it explicit (example below
 > works):
 > 
 > 
 > template <typename IntType>
 > inline rational<IntType> abs(const rational<IntType>& r)
 > {
 > #ifndef BOOST_NO_STDC_NAMESPACE
 >     // We want to use abs() unadorned below, so that if IntType is a
 >     // user-defined type, the name lookup rules will work to find an
 >     // appropriate abs() defined for IntType.
 >     //
 >     // We protect this with BOOST_NO_STDC_NAMESPACE, as some compilers
 >     // don't put abs into std:: (notably MS Visual C++) and so we can't
 >     // "use" it from there.
 >     // fbp : changed to explicit qualifier
 >     //    using std::abs;
 >     // Note that with normalized fractions, the denominator is always positive.
 >     return rational<IntType>(std::abs(r.num), r.den);
 > #else
 >     // Note that with normalized fractions, the denominator is always positive.
 >     return rational<IntType>(::abs(r.num), r.den);
 > #endif
 > }
 > 
 > 
 > If you need more of suct tricks, take a look at STLport (www.stlport.org).
 > There, I often had to resolve this problem in same namespace, which even required
 > proxy functions to be deployed.
 > 
 > 
 > Best regards,
 > -Boris.
 > 
 > Paul Moore wrote:
 > 
 > > 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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 > >
 > >   ------------------------------------------------------------------------
 > >                    Name: rational.hpp
 > >    rational.hpp    Type: C++ Header File (application/x-unknown-content-type-hppfile)
 > >                Encoding: quoted-printable
 > >
 > >                            Name: rational_example.cpp
 > >    rational_example.cpp    Type: C++ Source File (application/x-unknown-content-type-cppfile)
 > >                        Encoding: 7bit
 > 
 > 
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