Need a lambda friendly unordered_set like find()

[craigminihan]

The std::unordered_set does not support a lambda implementation of find(). This means the template type must be created to get a key comparison. In the key cache case the type is a shared_ptr which is a relatively expensive object. It is much preferred to compare the field count and the hash without the expense of creating a smart pointer instance.

[craigminihan]

There is also the nasty side effect that new must be invoked to create the key item in order to do the lookup - mitigated with an ugly shared_ptr null deleter.