Question

I have written the following code and was surprised at the output. I heard that <map> avoids collision of keys, but here it appears to allow insertion of duplicate pairs.

#include<iostream>
#include<map>

using namespace std;

int main()
{
    map<string,char> namemap;
    namemap["yogi"]='c';

    namemap.insert(pair<string,char>("yogendra",'a'));
    namemap.insert(pair<string,char>("yogendra",'b'));

    cout<<namemap["yogendra"]<<endl;

    return 0;
}

This code outputs a. You can run it on C++ Shell.

Does avoiding collision means that we cannot enter multiple pairs with same key?

Answer 1

The second insert with the same key is a no-op. It simply returns an iterator pointing to the existing element.

std::map::insert() has a return value, which you should check.

It is of type std::pair<iterator,bool>. The second element of the pair tells you whether the element has been inserted, or whether there was already an existing entry with the same key.

cout << namemap.insert(pair<string,char>("yogendra",'a')).second << endl;
cout << namemap.insert(pair<string,char>("yogendra",'b')).second << endl;