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From: kevin.cline@gmail.com (Kevin Cline)
Newsgroups: comp.lang.ada
Subject: Re: Ada Popularity: Comparison of Ada/Charles with C++ STL (and Perl)
Date: 20 Oct 2004 19:51:03 -0700
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Date: 2004-10-20T19:51:03-07:00
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"Robert Kawulak" <tigrisek@interia.pl> wrote in message news:<cl6gok$q0e$1@nemesis.news.tpi.pl>...
> Hi,
> 
> > #define is also parsed! In compiled languages "parsed" is far less than
> > "compiled" and the results of these two actions are much different too.
> 
>   OK, by saying 'parsed' I meant 'compiled'... Don't catch me in my
> words! ;-)
> 
> > The
> > margin is not absolute of course, but parsed template body is closer to
> > parsed #define body than to a compiled true body.
> 
>   Templates are not so close to #defines. Using the 'export', keyword their
> implementation may be *compiled* in only one translation unit, and you may
> use them anywhere given only their declarations  - just like ordinary
> functions and objects (well, at least in theory, because there are not so
> many compilers supporting 'export' yet... But I hope this will change soon).
> 
> > What if the *implementation*
> > does not fulfill the contract, while the parameter does?
> 
>   Ah, I see (at last ;-) what's your point. But is it really necessary to
> check if the *implementation* is contract-conformant? If it's not
> contract-conformant, then you get a compile-time error anytime you try to
> use it, the only difference I see is that such an error is detected only
> during instantiation. I know it'd be nice if the implementation would be
> checked for contract-conformance during compilation (parsing, whatever ;-),
> but OTOH I don't see not doing this such an issue...

Actually, this wouldn't be so nice.  As it is, member functions are 
instantiated individually, so we can write:

  template <class T> class Container
  {
    typedef ... const_iterator;
    void write(ostream& out) const
    {
      for (const_iterator i ... )
      {
         out << *i;
      }
    }
  }

and still be able to instantiate Container<X> even if
operator<<(ostream&, X)
is not defined.  There is no error unless we call Container<X>::write.

Of course, modern C++ experts would assert that functions like write
should be free functions and not class member functions.