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From: "kevin  cline" <kevin.cline@gmail.com>
Newsgroups: comp.lang.ada,comp.lang.c++,comp.realtime,comp.software-eng
Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: 14 Mar 2005 13:02:10 -0800
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Date: 2005-03-14T13:02:10-08:00
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Dr. Adrian Wrigley wrote:
> On Mon, 14 Mar 2005 20:43:08 +0200, Ioannis Vranos wrote:
>
> > Dr. Adrian Wrigley wrote:
> >
> >> the problem as I suggested in my previous post is that you have to
> >> pass in the comparison operator or hash function down the tree
> >> of template/generic instantiation, (composing them along the way)
> >> if you want to use maps/hashes.
> >
> > In the standard library containers and algorithms case you do not
pass
> > any function to the container. operator< is implicitly used.
>
> The comparison operator here seems to be a generic formal function
> parameter with a default value.  This must be available at
instantiation.
> The operator becomes part of the interface (implicitly or explicitly)
for
> any code instantiating the std::map.  Where does the code for the
> comparison operator go?  It *has* to go at the point the class
> is defined.

Actually, it doesn't.  If not defined as a member function in the
class, it can be defined as a free function wherever convenient.  Or a
custom comparison function can be defined and specified in the
instantiation.

Either this:

   class Stuff {
   public:
      bool operator<(const Stuff& rhs);
   };

or this:

   bool operator<(const Stuff& lhs, const Stuff& rhs);

will work with this:

   std::set<Stuff> allStuff;

Or you can define a special comparator to be used for a particular
instantiation, like this:

   struct compareStuff()
   {
       bool operator()(const Stuff& lhs, const Stuff& rhs) { ... }
   }

   std::set<Stuff, compareStuff> zeds;