Re: [boost] [density] new library

Il 13/06/2016 19:56, Klaim - Joël Lamotte ha scritto:

You might want to clarify the differences with this library as colony is well advanced in it's production. The main difference is that plf::colony is an homogeneous container (elements have all the same type), while density deals with heterogeneous containers, in which elements may have a common base class (i.e. polymorphic containers like dense_list<Widget>), or may be completely type-erased (for example: dense_list<void> or dense_function_list). The core idea of colony is avoiding element reallocation and moving. The core idea of density is avoiding wastes of memory and sparse data structures.

Another difference is in the memory management: plf::colony allocates increasingly big memory blocks while the container get filled, while density paged containers allocate fixed-size pages. Fixed size allocation is extremely fast and straightforward. Furthermore, a page allocator may use a thread-local temporary depot of free pages to get a new page without having to synchronize with the other threads. This optimization is not still implemented, but I believe that it will be beneficial for all the paged containers (that is paged_queue, paged_function_queue, and all the lifo classes). Anyway density is already performing better than the alternatives, especially with queues (http://peggysansonetti.it/tech/density/html/func_queue_bench.html). density deals with lifo allocation as well. This may may at first seem unrelated to heterogeneous containers, but actually it's not. For example you may need to copy or move on the stack an element from an heterogeneous container, but you don't know how big it is, so you use a lifo_buffer, like in the producer-consumer sample (http://peggysansonetti.it/tech/density/html/producer_consumer_sample.html). As explained in the comments, the drawback of using lifo_buffer in this way is that the code is not automatically exception safe. Very soon density will provide a type-erased lifo_object that will fix this. I care a lot about exception safeness: the test of density performs an exhaustive test on the strong exception guarantee on all the testcases, trying to raise an exception in every place in which an exception may occur and checking in a catch block that the container has not changed.