Re: using less<string>...

From:

Mark P <usenet@fall2005REMOVE.fastmailCAPS.fm>

Newsgroups:

comp.lang.c++

Date:

Sat, 13 May 2006 18:14:47 GMT

Message-ID:

<rmp9g.86404$dW3.66877@newssvr21.news.prodigy.com>

Martin JHrgensen wrote:

Hi,

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#include <iostream>
#include <string>
#include <map>
using namespace std;

int main()
{
    string name;
    int pop;

    string states[] = { "Wyoming", "Colorado", "Nevada",
        "Montana", "Arizona", "Idaho"};
    int pops[] = { 470, 2890, 800, 787, 2718, 944 };

    map<string, int, less<string> > mapStates;

// Try replacing the above with:
// map<string, int> mapStates;

    map<string, int, less<string> >::iterator iter; //iterator

    for(int j=0; j<6; j++)
        {
        name = states[j]; //get data from arrays
        pop = pops[j];
        mapStates[name] = pop; //put it in map
        }
    cout << "Enter state: "; //get state from user
    cin >> name;
    pop = mapStates[name]; //find population
    cout << "Population: " << pop << ",000\n";

    cout << endl; //display entire map
    for(iter = mapStates.begin(); iter != mapStates.end(); iter++)
        cout << (*iter).first << ' ' << (*iter).second << ",000\n";
    return 0;
}

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The question is: I don't see any difference between:

    map<string, int, less<string> > mapStates;

and

    map<string, int> mapStates;

There is no difference. The default Comparison for a map is
std::less<Key> so you've simply explicitly provided the default parameter.

I tried to google around a bit and found that both lines call the map
constructor to create a map<string, int> and the less<string> is a
comparison function - a 'weak ordering'. Can I see how less<string> is
defined somewhere?

You can look at your implementation's definition although in the end
you'll find it's equivalent to operator<.

-Mark