Re: [boost] flyweight - gsoc idea - multikey-value flyweight

Le 16/02/2016 16:00, Joaquin M LópezMuñoz a écrit :

Vicente J. Botet Escriba writes:

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Le 15/02/2016 07:51, Joaquin M LópezMuñoz a écrit :

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Vicente J. Botet Escriba writes:

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Hi Joaquin,

I have a use case where I have several keys to the same flyweight. [...]

Let say you are writing an application that manage with some entities. You have two kind of operator that use a specific protocol and each protocol identify these entities using a key MOi. Suppose that you application use a library that has an interface that use different keys to identify these same entities, because it is using an internal protocol. You want also to have some CLI for debugging purposes that uses strings to identify these same entities.

All these keys are isomorphic so you can move from one key to the other using a specific algorithm[...] OK, let's sketch these ideas. Let's say

<snip>

All of this allows us to do

int main() { multi_flyweight f1{124}; multi_flyweight f2{char{124}}; multi_flyweight f3{f2.get()}; multi_flyweight f4{f1};

std::cout<

with output

key2_from_key1 value_from_key1 124 124 124 124 1 1 1

which effectively shows that key and value creation is minimized, and the resulting semantics is what we were after. You can play with a complete example at

http://coliru.stacked-crooked.com/a/6dab2c556433fb03

Does this approach your use case? Form the functional point of view, yes, this is what I want. However we need a generic class and ensure that the size is really fly weight. Some points:

* This can be easily generalized to N keys with some metaprogramming. I believe even for 2 key we will need some meta-programming. * Note that we haven't changed the internals of Boost.Flyweight to use something like a multi-key factory (which is probably what you had in mind). Instead, this is all done on top of the existing library, which has the obvious drawback (among others) that sizeof(multi_flyweight)==2*sizeof(void*), where sizeof(flyweight) is typically sizeof(void*). Whether this is acceptable or not depends really on the use cases, I'd say. No, I want really a fly weight. If I have N key the multi_flyweight will not be fly weight anymore.

sizeof(multi_flyweight)==N*sizeof(void*)

* Rewriting this in the most efficient way through multi-key factories (likely based on Boost.MultiIndex) can be done but requires introducing whole new concepts into the framework --current factories are by definition one-key, so they simply can't be used for this. Agreed. Maybe, we can have a separated multi_flyweight class that takes care of this additional case. I see that the implementation is already using multi-index ;-) * Is this a desireable extension to Boost.Flyweight? I don't have a strong opinion myself, others' most welcome. I have no problem if this is in another library or class. What interest me is the functionality with the best performances in space and time. * Is this a good candidate for GSoC? If the answer to the above is yes, I'd say this is certainly in the good ambition/impact ratio window for a GSoC project, and I'd be happy to mentor. Glad to hear that. I believe this makes a good GSoC project. I could help you if needed. Do you mind to add an entry on the wiki https://svn.boost.org/trac/boost/wiki/SoC2016?

Best, Vicente