Re: map with pair as key

From:

Mark P <usenet@fall2005REMOVE.fastmailCAPS.fm>

Newsgroups:

comp.lang.c++

Date:

Tue, 20 Jun 2006 19:01:23 GMT

Message-ID:

<7CXlg.152240$F_3.25457@newssvr29.news.prodigy.net>

kietzi@web.de wrote:

Hello world,
I was wondering whether it would be possible to create a map which uses
a pair of ints as key and a float as value. I have used maps with const
char*[] as key, but have so far not been successful to use a version
which uses pairs as keys.

For instance, I would like to set up the map as follows:

struct eqstr {
    bool operator()(pair<int,int> s1, pair<int,int> s2) const{
        return ((s1.first==s2.first) && (s1.second==s2.second));
    }
};

std::map<std::pair<int,int>*, float, hash<const char*>, eqstr>
connections;

First of all, hash is not standard C++.

More to the point, your map template parameters look to be out of whack.
  The template parameters should be map<Key,Type,Compare,Alloc>. I'm
not sure how hash is supposed to fit into this scheme but what you need
is a less than function for the Key type. And clearly eqstr is not an
allocator-- most likely you don't need to specify this fourth parameter
at all.

For example...

struct PairCompare // or, ComPair :)
{
   bool operator () (pair<int,int>* pp1, pair<int,int>* pp2) const
   {
     return pp1->first < pp2->first ||
            pp1->first == pp2->first && pp1->second < pp2->second;
   }
};

-Mark