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Date: Mon, 14 Mar 2005 21:17:39 +0100
From: Georg Bauhaus <bauhaus@futureapps.de>
Organization: future apps GmbH
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Newsgroups: comp.lang.ada,comp.lang.c++
Subject: Re: [OT] Re: Teaching new tricks to an old dog (C++ -->Ada)
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Date: 2005-03-14T21:15:25+01:00
List-Id: <comp.lang.ada>

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. 

I think this description is o.K. for Ada, but perhaps not
for C++. E.g. you can place the following in three different
files (modules), somewhat like in Ada, and you don't have
to write the < into the module containing the compared type.

// 1

#include "comp.h"
#include <map>

bool operator< (V a, V b) { return a.val() < b.val(); }
  
int main()
{
  std::map<V, float> m;
  V key1, key2 = V(3);

  m[key1] = 1.0;
  m[key2] = 1.0;
 
  return 0;
}

// 2

class V {
  int v;
public:
  V() : v(0) {}
  V(int val) : v(val) {}
  int val() const;
};

// 3

#include "comp.h"

int V::val() const {
  return v;
}

Georg