I noticed when applying the remove/erase idiom that the following method is not implemented:

iterator erase ( const_iterator first, const_iterator last );

Reference: https://en.cppreference.com/w/cpp/container/unordered_map/erase
Example:
cache.erase( std::remove_if( cache.begin(), cache.end(), [&] ( const pair<int,int>& i ) { return i.second == 1 ); } ), cache.end() );