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From: =?ISO-8859-1?Q?Falk_Tannh=E4user?= <falk.tannhauser@crf.canon.fr>
Newsgroups: comp.lang.ada,comp.lang.c++,comp.realtime,comp.software-eng
Subject: Re: [OT] Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Mon, 14 Mar 2005 17:06:10 +0100
Organization: Canon Research Centre France
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Date: 2005-03-14T17:06:10+01:00
List-Id: <comp.lang.ada>

Dr. Adrian Wrigley wrote:
> I have probably missed a trick in the C++, but I couldn't get
> std::map code to compile (except in the trivial cases):
> 
> #include <map>
> 
> struct compoundindex {
>   int a, b, c;
> };
> 
> int main()
> {
>     std::map<compoundindex, float> hash;
>     compoundindex fred = { 1, 2, 4 };
> 
>     hash[fred] = 0.123;
> }

C++ associative containers (map, set, multimap, multiset) require
a "strict weak ordering" for their key types. In the present case,
it is enough to define the following 'operator<' to induce a total
ordering:

   bool operator<(compoundindex const& x, compoundindex const& y)
   {
     if(x.a < y.a) return true;
     if(x.a == y.a)
     {
       if(x.b < y.b) return true;
       if(x.b == y.b)
         return x.c < y.c;
     }
     return false;
   }

Alternatively, if there is no natural definition for a general-purpose
operator< for the key type in question, it is possible to create a
comparison functor and to instantiate the container with it as
supplementary template parameter.
In either case, the container implementation uses the order to store its
elements in a balanced tree, so that element access becomes possible
with logarithmic complexity regarding to the number of elements currently
in the container.

Hash-based associative containers (guaranteing constant-time access as
long as there are no key collisions) are not (yet) provided by the
standard library but are widely available through third party / extension
libraries.

Falk