std::unordered_map and duplicate keys

Question

I'm using an stl unordered_map, and I can't seem to get the count method to work. This is my program:

typedef unordered_map<char, int> Mymap;
int main() 
{
    Mymap m;  

    m.insert(Mymap::value_type('a', 1)); 
    m.insert(Mymap::value_type('b', 2)); 
    m.insert(Mymap::value_type('c', 3)); 
    m.insert(Mymap::value_type('b', 4)); 
    m.insert(Mymap::value_type('b', 5)); 

    cout << m.count('b') << endl;

    return 0; 
} 

The documentation for unordered_map says that unordered_map::count(const Key& k) returns the number of elements with the key k. So I would expect the output here to be 3, whereas the real output is 1. Why?

Answer 1

An unordered_map maintains a 1:1 mapping of key to value, so count will always return zero or one.

You need an unordered_multimap if you want to map multiple values to a single key.