JOAQUIN LOPEZ MU?Z ha scritto:
I'm glad you've raised an important issue here. As it happens, during the design of the library I devoted some thinking to the map-vs-set problem (if you allow me to call it that way) and I opted for the current design for the following reasons:
<snip>
2. Indeed GoF introduces a key type K into the pattern that is used to retrieve the actual values of T. So, we have a one-to-one relation K-->T, i.e. there exists a stateless function f of the form
T f(const K&);
that can be used to construct a T from a given K. And, additionally, K is cheaper to construct than T. This is the map approach, right? My question now is: is this a realistic scenario? If K is actually cheaper to construct than T and we can univocally get the associated T from any K, why work with Ts and not just use Ks in the first place? The only plaussible justifications I can think of is that f() is computationally expensive or that T is more convenient to work with than K, but these seem (to me) not so likely concerns --more on the second concern on point 3 below, though. Note that I explicitly observed that f must be *stateless*, i.e. a K object contains exactly the same information as its associated T value. This is not the case in most usages of std::map, which is a reason why std::maps are useful :) I am not plainly denying the existence of sensible K-->T scenarios, but I thought long and hard and couldn't find any. If you can come up with one I'll be happy to know. So, my analysis led me to conclude that the right approach is to assume that K==T, that is, the set approach, or at most than K and T are just different representations of the same information.
One such scenario is right there in the GoF book, the word-processing application that uses one flyweight object for each glyph in the document. I have another case in an project I am working on: in a 3D application, I have heavyweight 3D mesh objects that might be shared among several actors. The key of a 3D mesh is just its filename. I don't want to load an entire mesh into memory just to find out that I have it already. Yes, I could delay the actual loading of the mesh until the first time it is actually used, but that would be impractical for at least two reasons: 1) any error encountered while loading the mesh would occur in the wrong place and couldn't be handled properly, 2) the place where the mesh is used is inside a tight rendering loop with strict real-time requirements which can't be blocked by costly I/O operations.
3. That said, there is a natural evolution for the library that would eliminate T construction under some circumstances: C++0x containers will provide the emplace() member function allowing for in-place construction of an inserted elements given the ctor arguments, so eliminating the need to construct a temporary to pass to insert() and related memfuns. When this is available, the flyweight lib will take advantage of it, so that
fw s2("foo");
will create no temporary std::strings.
Actually it will always create one temporary. If the string is not already present in the factory, the temporary is then moved into the factory and so it is not created in vain. However, in the most common case, the string is already present in the factor and the temporary needs to be discarded. The main problem is that in order to avoid the creation of the temporary you would need to be able to compare keys with objects directly, while the current design requires constructing an object in order to compare it with the existing objects. In this sense, I believe the find/insert approach might help addressing the issue.
will create no temporary std::strings. In the terminology of point 2, we have K==const char*, T==std::string. You can argue that this seems to contradict the thesis of point 2 (there are no sensible usa cases where K!=T), but note that this situation (constructing a std::string from a const char*) is only marginally useful: in a real program most std::strings do not get constructed from compile time literals, but rather they are computed dynamically, so the optimizations we can introduce here will not dominate the overall performance of the program.
I may agree with that, but you should understand that strings are not the most general case. In the GoF example, glyphs are created from simple characters. The fact that they come from literals or are computed dynamically does not make a big difference, they're just characters. In such a scenario your framework couldn't avoid creating an unnecessary temporary glyph.
* What is your evaluation of the implementation? The implementation seems well done. The use of the "named parameters" idiom is very interesting.
However, I strongly disagree with the choice of the term "factory" for a component that actually only acts as a storage. In GoF the term factory is properly used for a component which is devoted to construct the final object given its identifying key. But in this proposal it's the user code that actually constructs the object, so the term is used incorrectly and is misleading.
I decided to keep "factory" because this component is obviously related to the element of the same name in GoF and oher descriptions of the pattern. Incidentally, when point 3 above gets implemented the factory will act more like a factory in the sense you describe. That said, I'll be happy to use whatever other terminology reviewers agree upon.
I would keep the "factory" term if you are considering the find/insert approach as an alternative to the current insert-only approach. With that approach case, find() would be given a key, rather than an object. If the object is not found, insert() would have to create an object out of the information stored in the key and that would actually be what factories are expected to do.
* What is your evaluation of the potential usefulness of the library? The library as it is, is not very useful, although it might be potentially very useful if the flaw I described before could be addressed.
I hope you might concede that the lib is useful in the scenarios described at point 1 above, even if these are not the modes of usage you envisioned in the frist place. Looking forward to your discussion of the arguments I presented here.
Maybe I was a little to harsh. I concede that. Having reduced memory usage and manipulating heavy-weight objects through lighter-weight handles is certainly an advantage over a naive approach. This advantage can be felt considerably once the flyweight "pool" is established and all that is left is to manipulate the objects. However, there are cases, for example the two cases I presented in this post, where the construction of the actual flyweight objects has a non negligible cost: in the GoF word-processing case just because of the very large number of glyphs, in the 3D mesh case because of the very high cost of I/O operations. In those cases, the current design makes the task of establishing the flyweight "pool" less efficient than hand-coded solutions based on maps. As in my projects I am very concerned with start-up times I view the proposed library design as mere syntactic sugar and I would definitely prefer the hand-coded solution. Just my opinion, Ganesh