+1 value approach (via some implementation from this thread) I think I like this.
In all BAD cases, it's the user who shot themselves in the foot by using std::move unsafely. I expect this behavior is the same behavior as for any other object. And if we're ever able to get rid of the Cache/CacheImpl circular dependency then we can add a copy constructor. I also like the idea of passing in a CacheConfig. My concern though is that it's piling on another big change. Thanks, David On Sun, Sep 17, 2017 at 12:02 AM, Jacob Barrett <jbarr...@pivotal.io> wrote: > Ok, one more idea. > https://gist.github.com/pivotal-jbarrett/beed5f70c1f3a238cef94832b13dab7a > > The biggest issue with the value model is that we have been using a factory > to build the Cache object. We really don't need one and if we get rid of it > things look much better. They aren't perfect since we still need the back > pointer from the impl to the cache for later use. If we didn't need that > then we could allow copy construction. As it stands right now this version > allows value, shared_prt, unique_ptr, etc. without any real overhead or RVO > issues. > > The big change is that, rather than have a factory that we set a bunch of > values on and then ask it to create the Cache, we create a CacheConfig > object and pass that in to the Cache's constructor. Cache passes it to > CacheImpl and CacheImpl sets itself up based on the config. If you look at > what the current factory model does it isn't that different. For clarity I > added an XmlCacheConfig object to that builds up the CacheConfig via Xml. > You could imagine a YamlCacheConfig object *shiver*. The point is we don't > care as long as we get a CacheConfig with all the things we support at > "init" time. > > I know it is a more radical change but I feel it is more C++ and more > testable than the factory model. I also like that it solves some of the > issues with the value model we were looking at. > > -Jake > > On Thu, Sep 14, 2017 at 5:16 PM Jacob Barrett <jbarr...@pivotal.io> wrote: > > > Y'all here is an attempt to get the best of both worlds. > > https://gist.github.com/pivotal-jbarrett/52ba9ec5de0b494368d1c5282ef188 > ef > > > > I thought I would try posting to Gist but so far not impressed, sorry. > > > > The Gist of it is that we can overcome the thirdpary or transient > > reference back to the public Cache instance by keeping a reference to it > in > > the implementation instance and updating it whenever the move constructor > > is called. > > > > The downside is if your run this test it doesn't show RVO kicking in on > > the second test where we move the value into a shared pointer. There are > a > > couple of pitfalls you can stumble into as well by trying to used the > > previous instance to access the cache after the move operation, as > > illustrated by the "BAD" commented lines. > > > > The upside is the choices this gives the use for ownership of their > > factory constructed Cache instance. They can keep it a value or move it > to > > unique or shared pointer. > > > > Overhead wise I think we better off in value as long as there are no > > moves, rare I would thing, but the moves are pretty cheap at the point > > since we only maintain a unique_ptr. After moving into a shared_ptr it > acts > > identical to the shared_ptr model proposed earlier. > > > > -Jake > > > > > > On Thu, Sep 14, 2017 at 3:36 PM Michael Martell <mmart...@pivotal.io> > > wrote: > > > >> Late to this party. > >> > >> Confession 1: I had to look up both RVO and copy-elision. > >> Confession 2: I don't like using pointers at all. I used to, but I've > >> evolved to just use C# and Java :) > >> > >> Without investing a lot more time, I don't have strong feelings about > raw > >> vs shared pointers. My only question is: Can we return ptr to abstract > >> class everywhere we return objects? Just thinking of mocking, which > always > >> wants to mock interfaces. > >> > >> On Thu, Sep 14, 2017 at 2:25 PM, Michael William Dodge < > mdo...@pivotal.io > >> > > >> wrote: > >> > >> > +0 shared pointer > >> > > >> > > On 14 Sep, 2017, at 14:09, Ernest Burghardt <eburgha...@pivotal.io> > >> > wrote: > >> > > > >> > > Calling a vote for: > >> > > > >> > > - Raw pointer > >> > > - shard pointer > >> > > > >> > > +1 raw Pointer, I had to look up RVO and am new to std::move(s) > >> > > > >> > > On Thu, Sep 14, 2017 at 3:02 PM, Michael William Dodge < > >> > mdo...@pivotal.io> > >> > > wrote: > >> > > > >> > >> I generally dig reference-counted pointers for avoiding lifetime > >> issues > >> > >> with objects allocated off the heap but I can live with bare > >> pointers, > >> > too. > >> > >> > >> > >> Sarge > >> > >> > >> > >>> On 13 Sep, 2017, at 16:25, Mark Hanson <mhan...@pivotal.io> > wrote: > >> > >>> > >> > >>> Hi All, > >> > >>> > >> > >>> I favor the “pointer" approach that is identified in the code > >> sample. > >> > >> There is greater clarity and less bytes seemingly created and > >> written. > >> > We > >> > >> do sacrifice the potential ease of using an object, but in all, I > >> think > >> > the > >> > >> way our code is structured. It is not conducive to do a value > >> approach, > >> > >> from an efficiency standpoint, because of our use of the pimpl > >> model. > >> > >>> > >> > >>> Thanks, > >> > >>> Mark > >> > >>> > >> > >>>> On Sep 12, 2017, at 11:09 AM, Jacob Barrett <jbarr...@pivotal.io > > > >> > >> wrote: > >> > >>>> > >> > >>>> My biggest concern with this model is that access to the public > >> Cache > >> > >>>> object from other public objects results in additional > allocations > >> of > >> > a > >> > >>>> Cache value. Think about when we are inside a Serializable object > >> and > >> > we > >> > >>>> access the Cache from DataOutput. > >> > >>>> > >> > >>>> As value: > >> > >>>> Serializable* MyClass::fromData(DataInput& dataInput) { > >> > >>>> auto cache = dataOutput.getCache(); > >> > >>>> ... > >> > >>>> } > >> > >>>> In this the value of cache will RVO the allocation of Cache in > the > >> > >> getCache > >> > >>>> call into the stack of this method, great. The problem is that > >> Cache > >> > >> must > >> > >>>> contain a std::shared_ptr<CacheImpl> which means that each > >> allocation > >> > >> is 8 > >> > >>>> bytes (pointer to control block and pointer to CacheImpl) as well > >> as > >> > >> having > >> > >>>> to increment the strong counter in the control block. On > >> exit/descope, > >> > >> the > >> > >>>> Cache will have to decrement the control block as well. > >> > >>>> > >> > >>>> Using current shared_ptr pimple model: > >> > >>>> Serializable* MyClass::fromData(DataInput& dataInput) { > >> > >>>> auto& cache = dataOutput.getCache(); > >> > >>>> ... > >> > >>>> } > >> > >>>> We only suffer the ref allocation of 4 bytes and no ref count. > >> Since > >> > >> this > >> > >>>> function call can't survive past the lifespan of Cache/CacheImpl > >> they > >> > >> don't > >> > >>>> need to have shared_ptr and refcounting. > >> > >>>> > >> > >>>> Given that this method could be called numerous times is the > >> overhead > >> > of > >> > >>>> the value version going to be a significant performance issue? > >> > >>>> > >> > >>>> I worry that moves and RVO is just beyond most developers. Heck I > >> > didn't > >> > >>>> know anything about it until we started exploring it. > >> > >>>> > >> > >>>> > >> > >>>> > >> > >>>> -Jake > >> > >>>> > >> > >>>> > >> > >>>> On Tue, Sep 12, 2017 at 8:06 AM David Kimura <dkim...@pivotal.io > > > >> > >> wrote: > >> > >>>> > >> > >>>>> Follow up of attached discussion after more investigation. I > >> created > >> > >> an > >> > >>>>> example of returning Cache as shared pointer versus raw value: > >> > >>>>> > >> > >>>>> https://github.com/dgkimura/geode-native-sandbox > >> > >>>>> > >> > >>>>> I still like returning by value as it lets the user do what they > >> want > >> > >> with > >> > >>>>> their object. > >> > >>>>> > >> > >>>>> // Here user creates object on their stack. > >> > >>>>> auto c = CacheFactory::createFactory().create(); > >> > >>>>> > >> > >>>>> // Here user creates smart pointer in their heap. > >> > >>>>> auto cptr = > >> > >>>>> std::make_shared<Cache>(CacheFactory::createFactory(). > create()); > >> > >>>>> > >> > >>>>> Difficulty of implementing this is high due to circular > >> dependencies > >> > of > >> > >>>>> Cache/CacheImpl as well as objects hanging off CacheImpl that > >> return > >> > >>>>> Cache. We must be extra careful when dealing with move/copy > >> > semantics > >> > >> of > >> > >>>>> Cache/CacheImpl. > >> > >>>>> > >> > >>>>> Alternative, is to keep as is and only permit heap allocations > >> from > >> > >>>>> factory using shared pointers. > >> > >>>>> > >> > >>>>> Thanks, > >> > >>>>> David > >> > >>>>> > >> > >>> > >> > >> > >> > >> > >> > > >> > > >> > > >
