Re: [Boost-Users] Query regarding the use/limitiations of traits in metaprogramming

From: Noel Yap (yap_noel_at_[hidden])
Date: 2002-05-07 11:19:31

It looks like it's working find with gcc-2.95.3-5. 
One thing, though, it gave me a compiler error when I
didn't #include <typeinfo>.  Since you didn't list
which header files you're including, I'm not sure if
you're including <typeinfo> or not and what the
Borland's behaviour would be if you didn't.

Noel
--- hicks <hicks_at_[hidden]> wrote:
>   Hello
> 
> 
> >I have some questions for you:
> >1. Why are you so interested in the value of
> >"typeid.name()"?
> 
> I'm not particulary interested in typeid::name() as
> an end result.
> As I said in the letter, I have real task at hand,
> and condensed the 
> code so that anybody looking at it would be able to
> get to the point quickly.
> I used typeid()::name() here as a means of
> demonstating the problem.
> The result which is shown by typeid()::name()
> corresponds to the 
> result encountered in the real task, namely,
> the next stage of processing is supposed to take
> place in 
> traits1<int, 1> but takes place in traits1<int, 0>
> instead.
> In other words, the branching mechanism fails
> silently.
> 
> 
> >2. Are you sure this isn't a compiler problem? 
> What
> >compiler(s) have you tried this on?
> 
> 
> Borland Builder C++ 5.
> Unfortunately, it's the only one to which  I have
> access now.
> It could well be a compiler problem.  
> Unless there are some rules which I don't
> understand.
> If it is a compiler problem perhaps someone knows of
>  a workaround.
> I have tried various approaches to no avail.
> 
> Craig Hicks
> 
> Noel Yap wrote:
> 
> >--- hicks <hicks_at_[hidden]>
> <mailto:hicks_at_[hidden]> wrote:
> >
> >>Query regarding the use/limitiations of traits in
> >>metaprogramming
> >>
> >>I am trying to use traits pattern to assist in
> >>metaprogramming.
> >>I have the following problem.
> >>
> >>A traits class "trait" defers a decision to
> another
> >>traits class 
> >>"trait1" depending
> >>on the result of a third traits class "traita".  
> >>However, the compiler does not cooperate.
> >>The code below excutes but gives unexpected output
> >>(also shown).
> >>
> >>[Note:: The code has been reduced to the minimal
> to
> >>demonstrate the problem.
> >>I have a real task at hand not shown here.]
> >>
> >>template <class T>
> >>struct traita { BOOST_STATIC_CONSTANT(int,
> value=0);
> >>};
> >>template <>
> >>struct traita<int> { BOOST_STATIC_CONSTANT(int,
> >>value=1); };
> >>
> >>template <class T, int>
> >>struct trait1 {};
> >>
> >>
> >>template <class T>
> >>struct trait
> >>{
> >>    BOOST_STATIC_CONSTANT(int,
> >>value=traita<T>::value);
> >>    typedef trait1<
> >>;T, value> t1;
> >>};
> >>
> >>
> >>
> >>int main(int argc, char* argv[])
> >>{
> >>    std::cout
> >>    << typeid(trait<int>::t1).name()
> >>    << std::endl
> >>    << trait<int>::value
> >>    << std::endl
> >>    ;
> >>    return 0;
> >>}
> >>
> >>output::
> >>
> >>trait1<int,0>
> >>1
> >>
> >>It would be nice if the first line of output were
> >>"trait1<int, 1>", but 
> >>it isn't.
> >>
> >>Any assistance would be greatly appreciated.
> >>
> >
> >I have some questions for you:
> >1. Why are you so interested in the value of
> >"typeid.name()"?
> >2. Are you sure this isn't a compiler problem? 
> What
> >compiler(s) have you tried this on?
> >
> >Noel
> >
> >
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> 
> 
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