
Boost : 
From: jsiek_at_[hidden]
Date: 19991214 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
> // userdefined 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 userdefined 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 "integerlike" semantics, but it
> > blows any hope of optimisation for a userdefuined IntType right out of the
> > water. (If someone can offer me a suitable implementation for
> > standardconforming 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
> >
> > 
> > Check out Media Grok and 14 other FREE newsletters from
> > The Industry Standard. http://clickhere.egroups.com/click/1762
> >
> >  Talk to your group with your own voice!
> >  http://www.egroups.com/VoiceChatPage?listName=boost&m=1
> >
> > 
> > Name: rational.hpp
> > rational.hpp Type: C++ Header File (application/xunknowncontenttypehppfile)
> > Encoding: quotedprintable
> >
> > Name: rational_example.cpp
> > rational_example.cpp Type: C++ Source File (application/xunknowncontenttypecppfile)
> > Encoding: 7bit
>
>
> 
> GRAB THE GATOR! FREE SOFTWARE DOES ALL THE TYPING FOR YOU!
> Gator fills in forms and remembers passwords with NO TYPING at over
> 100,000 web sites! Get $100 in coupons for trying Gator!
> http://clickhere.egroups.com/click/2092
>
>  Create a poll/survey for your group!
>  http://www.egroups.com/vote?listname=boost&m=1
>
>
>
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk