Re: std::map lookup function
Matthias Kluwe wrote:
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.
Don't we all.
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.
I generally return a pointer to the mapped object -- null if it
isn't 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?
What you just wrote. I don't see where you get the reference to
the local object it. That object is an iterator; calling the
operator-> function returns a pointer to the actual object in
the map, and it is a reference to that object that you are
returning.
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