Let's say I have two maps:

typedef int Id;
std::map<Id, std::string> idToStringMap;
std::map<Id, double>      idToDoubleMap;

And let's say I would like to do a set operation on the keys of the two maps. Is there an easier way to do this than to create a custom "inserter" iterator? such that I could do something like:

std::set<Id> resultSet;
set_difference( idToStringMap.begin(), idToStringMap.end(), 
                idToDoubleMap.begin(), idToDoubleMap.end(), resultSet.begin() );

My experimentation results imply that it will be necessary to create a custom inserter and perhaps a custom key comparer to do this, but I want for some insight/shortcut before doing so.