STL <map> allows duplicate pairs?

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; 

Answer 2

STL map does not allow same Keys to be used. You may want to go for multi-map for that.