This is not a review of the library, rather a wish list. (I started to read it only now.) 1. (Not important) I personally would like a different name: "const_singleton" so. 2. There should be macro BOOST_FLYWEIGHT_NOT_USED which would disable functionality of the library so that flyweight<std::string> would become std::string - for troubleshooting and performance tests. An ability to disable flyweightness per type or tag would be useful. 3. More support for troubleshooting. I would like to see something as macro BOOST_FLYWEIGHT_PARANOIC_MODE where each factory instantiates 2 objects with the same value and the holder returns one or the other, alternating them over and over where possible. This should help to detect wrong use of the flyweight. 4. Support for unit tests run within application session. I would like to see something as: TEST() { ToolThatRevertsHolderOnBlockExitToStateAsNow<std::string> aux; ... use flyweight, e.g. create zillion different values in a stress test } Possibly an ability to temporarily preserve current instances owned by the specified holder, make the holder emply and on exit from the unit test restore the initial state. Ideally, I should have a way to do this with _all_ flyweight holders in the applications, w/o need to specify them explicitly. 5. (This is a biggie) Ability for the end user to specify somewhere the set of allowed shared values created by the factory, by enumeration or with a constraint on those values (only the surnames "Smith", "Jones", "Jackson" and those starting with "Mc") so all other values would be created and used as nonshared. The effects would be: * reduction of the possible unlimited growth of the stored instances, * locking and reference counting overhead for values too hard to share would be eliminated. 6. An API (possibly just in debug mode) which provides usage statistics for a flyweighted type (where available). 7. For people who learn from examples: the more of them the better. The documentation could have a section about potential disadvantages of flyweights. /Pavel