map vs multimap -- whats the difference?

[Halsafar]

Okay I though multimap was to map multiple values to one key and map was used to map 1 key to 1 value.


It seems that is not true.
Infact as I've been playing with map and multimap it seems you can insert multiple values for 1 key in both of them...

cppreference.com shows that maps are for key/value pairs and that multimap allows duplicate keys...

In both map and multimaps if I insert(1,2) then insert(1,3) i can get access to both 2,3 via 1

[CornedBee]

That's impossible. A map will overwrite the existing value if you insert another with the same key.

[Halsafar]

Untrue, snip the following into a workspace:

map<int, int> Map;
Map.insert(pair<int, int>(1,2));
Map.insert(pair<int, int>(1,3));

map<int, int>::iterator itr = Map.lower_bound(1);
cout << itr->second << endl;
cout << ++itr->second  << endl;

system("PAUSE"); 

The output is:
2
3

[CornedBee]

Non-portable random behaviour.

int main() {

        map<int, int> Map;
        Map.insert(pair<int, int>(1,2));
        Map.insert(pair<int, int>(1,3));

        map<int, int>::iterator itr = Map.lower_bound(1);
        cout << itr->second << endl;
        ++itr;
        if(itr == Map.end()) {
                cout << "Thought so." << endl;
        } else {
                cout << itr->second  << endl;
        }

}

Output:

2
Thought so.

Dereferencing the end() iterator yields undefined behaviour.

[Halsafar]

Well okay.
How come the first output is 2 then on my snippet?

I figured key 1 == value 3 since it is the last to be inserted.

[CornedBee]

Because insert() doesn't overwrite, I was wrong about that detail. It simply doesn't insert if the key already exists.
http://msdn.microsoft.com/library/de...fmapinsert.asp