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

From: ÒÝÁØ Ñî (yyl_20050115_at_[hidden])
Date: 2020-07-31 10:30:38

About
    void Dispose() {
        if (this->_ptr != nullptr) {
            PTR* _ptr = this->_ptr;
            this->_ptr = nullptr;
            _ptr->~PTR();
            RelRef(_ptr);
        }
}
Especially
       _ptr->~PTR();

This is not
delete _ptr;

Because:
      delete _ptr = _ptr->~PTR() + free(_ptr)
We use
      _ptr->~PTR()
to give shared_ptrs to decrease count and all descendent objects to decrease count, but not free any of them


And then check if anything to be free at the end of destructor of shared_ptr¡£
if anything including self to be free, we free them all together then.

    ~rcgc_shared_ptr() {
        this->Dispose();
        if (_ac) {
            Collect();
        }
}
If _ac (auto collect) is true, we do collect here.

void rcgc_base::Collect(std::vector<void*>& p_wilds)
{
    if (p_wilds.size() > 0) {
        for (auto p = p_wilds.begin(); p != p_wilds.end(); ++p) {
            if (*p) {
                //delete p->first;
                //NOTICE: use free instead of delete to avoid double calling destructor
                free(*p);
            }
        }
        p_wilds.clear();
    }
}

So we have options now: either collect at the end of destructor of shared_ptr, and collect everything with rc of 0;
or we collect everything when we need to collect or in other dedicated thread (that¡¯s the advantage).

BR,
Yilin


·¢ËÍ×Ô Windows 10 °æÓʼþ<https://go.microsoft.com/fwlink/?LinkId=550986>Ó¦ÓÃ

·¢¼þÈË: Andrey Semashev via Boost<mailto:boost_at_[hidden]>
·¢ËÍʱ¼ä: 2020Äê7ÔÂ31ÈÕ 17:51
ÊÕ¼þÈË: boost_at_[hidden]<mailto:boost_at_[hidden]>
³­ËÍ: Andrey Semashev<mailto:andrey.semashev_at_[hidden]>
Ö÷Ìâ: Re: [boost] Hello everybody, I have something to share about the shared_ptr class

On 2020-07-31 11:52, ÒÝÁØ Ñî 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.

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