Re: Hello everybody, I have something to share about the shared_ptr class

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2020-07-31 09:51:22

On 2020-07-31 11:52, 逸霖 杨 via Boost wrote:
> Hello dear boost members,
> 
> I’m quite honored to join this mail list for the first time!
> 
> As I was told that the boost project is where the shared_ptr class originated, so I think
> it should be OK to talk about it here.
> 
> Here I have some ideas and code on my version of shared_ptr class.
> 
> The aim is to remove the usage of weak_ptr and fix the circular reference problem.
> 
> Please check code on
> https://github.com/yyl-20020115/RCGC
> 
> That’s the idea and primitive implementation.
> I don’t know exactly how to test it more widely among situations, please help me to find
> the very situations that this solution would fail, and I’ll try to fix it ASAP.
> 
> READM.md is a good start and README_ZHCN.md is better if you can read Chinese.
> However, I don’t think I can illustrate the algorithm very well both in English and Chinese.
> Well the code speaks for itself, and please read the code and run the program.
> 
> Please generate a makefile and build with CMakeLists.txt on *nix platforms if Visual Studio 2019
> (where I code the project) is not available.
> 
> All ideas and discussions and help from you are all very welcome!

So, it looks like you're just doing garbage collection in a separate 
thread instead of freeing memory on the last reference counter 
decrement. Which means that your claim that memory consumption is the 
same as the original shared_ptr is not quite true. IMHO, GC is a too 
costly and unpredictable solution. I don't see myself ever wanting to 
have a GC, unless in a *very* localized and controlled context, like a 
single function cleaning up after itself.

Regarding how circular dependencies are resolved, I don't quite see the 
solution. In the rcgc_shared_ptr::Dispose method, you immediately 
destroy the referenced object, not even when the reference count drops 
to zero, which is plain wrong because other pointers may still refer to 
the object. If you didn't destroy it, in the circular dependencies case 
the reference count would never drop to zero, which means the GC thread 
would never collect the object.

Lastly, I question the usefulness of a shared_ptr without a weak_ptr. 
Note that weak_ptr is not limited to just circular references 
resolution, it has other use cases. Although shared_ptr has other useful 
features, like pointer aliasing and deleter erasure, I find myself using 
shared_ptr almost exclusively when I also need weak_ptr. When I don't 
need weak_ptr I would use intrusive_ptr. But then your rcgc_shared_ptr 
doesn't provide any of the additional features of shared_ptr, so there's 
even less reason to use it.