Re: std::map lookup function

Hi!

In my current (toy) project I need to lookup values in a std::map. As
this is constructed from user input, given keys can't be trusted, and I
find myself doing things like

    std::map<...,...>::iterator it = map.find( key );
    if ( it != map.end() ) {
        ...
    }

very often.

I'd like to implement a function which does the lookup, returns a
reference to the mapped object and throws an exception if the given key
is not present.

My first try was

struct Lookup_Error {};

template<typename Map>
typename Map::mapped_type&
lookup( Map m, const typename Map::key_type v ) {
    Map::iterator it = m.find( v );
    if ( it == m.end() ) throw Lookup_Error();
    return it->second;
}

Unfortunately, this returns a reference to a part of the local object
"it", so this won't work.

What is the solution?