Convenience function for const map read

[nobody]

It's really awkward to use an STL map when you have
only a const reference to it, because you cannot use
operator[] -- you have to use find, check if the value
was found, etc.  (The reason being that operator[]
creates a new map entry initialized with the null value
if it doesn't find the key.)  It would be much more
convenient to have
a lookup convenience function that either throws an
exception if the key doesn't exist, or returns a
default-initialized value.  The version that returns a
default value if the key doesn't exist might look
something like this:

template <class Map>
typename Map::data_type const &
map_get(Map const & m, typename Map::key_type key)
{
  static typename Map::data_type default_value;
  typename Map::const_iterator it = m.find(key);
  return (it == m.end() ? default_value : it->second);
}