"John Maddock" <john@johnmaddock.co.uk> wrote in message news:006a01c6403f$b0760640$caed1b52@fuji...
That's an implementation detail. It isn't required by the spec, although that may be the most obvious way to implement the spec. An alternate implementation would be to keep a pool of directory entry objects and recycle them if performance was a concern. It would be great if Boost had a cache library to make such a strategy trivial to implement.
What kind of cache did you have in mind? Regex has an "Object Cache" (see boost/regex/pending/object_cache.hpp) that I always meant to submit for full Boost status but never got around to :-(
It simply maps a "key" to an instance of an object constructed from the key: if the object already exists it returns it, otherwise it constructs a new one and caches it. Objects are returned as a shared_ptr<const Object> (the shared_ptr is needed for memory management, since neither the cache not the client have exclusive ownership on the object). Useage is simply:
shared_ptr<const Object> obj = object_cache<Key, Object>::get( my_key, max_number_of_objects_to_cache);
To be honest usefulness is pretty limited, and threading issues add a small performance hurdle, that means that "Objects" have to be fairly expensive to construct before it becomes useful.
Still, there you go ;-)
Rather than a cache that does A or a cache that does B, I'd rather see someone do a review of various cache features in the literature (or at least whatever Google can find), identify the most useful ones, and then design one or more classes (probably template based) that delivered that useful feature set. For example, "Least Recently Used", "Least Frequently Used", and "Least Recently Added" are common replacement policies, but there are probably some others that are occasionally useful. I've used a scheme where entries are never flushed as long as there are any live references to them. Another way to approach a cache library design would be to collect a set of use cases, and then see what feature set would be needed to satisfy those use cases. Caching is one of the fundamental ways to achieve improved performance. I'm a bit surprised it hasn't been more of a topic in Boost discussions, or in computer science in general. That isn't a criticism of your object_cache; it is just that I think the topic deserves more than a single-solution approach. --Beman