Hash map with concurrent values (in C++)

Question

I have a hash map, from a string key to a pointer to a thread-safe class. Each object of this class contains a mutex which I use to synchronize between its various methods.

In addition, every once in a while I want to discard values that were not used for some time. I want to be able to safely delete the value pointer and remove it from the hash map, while making sure that no one uses my value.

What's the best way to achieve this (in general, and in C++)?

Answer 1

If you want people to use your hash map to get these pointers then be able to keep using them without having to notify your hash map class that they've finished with it, then storing shared pointers in the hash map is the easiest way to time actual object deletion to the time the last user of the object finishes with it. The hash map is then free to erase it's shared pointer to the object at any time.

Answer 2

Store and pass around shared pointers (std::shared_ptr) to your objects. Use std::shared_ptr::use_count method to check how many pointers are being used at the moment

Answer 3

You can also use concurrent_hash_map from TBB library. It protects concurrent accesses and lifetime of its elements using accessors (kind of a smart pointer with a lock)