I need to have a key with multiple values. What datastructure would you recommend?

std::multimap

The link provides an excellent example. Quoted below:

 int main()
{
  multimap<const char*, int, ltstr> m;

  m.insert(pair<const char* const, int>("a", 1));
  m.insert(pair<const char* const, int>("c", 2));
  m.insert(pair<const char* const, int>("b", 3));
  m.insert(pair<const char* const, int>("b", 4));
  m.insert(pair<const char* const, int>("a", 5));
  m.insert(pair<const char* const, int>("b", 6));

  cout << "Number of elements with key a: " << m.count("a") << endl;
  cout << "Number of elements with key b: " << m.count("b") << endl;
  cout << "Number of elements with key c: " << m.count("c") << endl;

  cout << "Elements in m: " << endl;
  for (multimap<const char*, int, ltstr>::iterator it = m.begin();
       it != m.end();
       ++it)
   cout << "  [" << (*it).first << ", " << (*it).second << "]" << endl;
}

you can use a multimap from the STL and use the call

pair<iterator, iterator> equal_range(const key_type& k)

to get a range of iterators that match your key

personally i find this slightly clunky due to having to deal with iterator ranges rather than just getting an object back that represents all values for that key. to get around that you could also store a vector in a regular map and add your strings to the vector.

If you're using C++ then just create a class to represent your key-value pairs:

Class foo {
    key : String
    values : list of values
}

Then, create a map that maps each key to an object containing its values.

This is simple, extendable, and can be done in any OO-language.

Sorry, my C++ is rusty so the syntax is wrong, but the essential idea is straightforward.