Re: [boost] [smart ptr] Any interest in copy-on-write pointer for C++11?

Subject: Re: [boost] [smart ptr] Any interest in copy-on-write pointer for C++11?
From: Ralph Tandetzky (ralph.tandetzky_at_[hidden])
Date: 2013-02-08 12:52:51

On 02/08/2013 06:20 PM, Mathias Gaunard wrote:
> On 08/02/13 17:50, Ralph Tandetzky wrote:
>> It is as thread-safe as necessary. The reference counter is atomic. It's
>> all well documented on github
>> <https://github.com/ralphtandetzky/cow_ptr>. You can find all the
>> thread-safety guarantees there. Thread-safety is documented at the
>> bottom of the long comment preceeding the cow_ptr class.
>
> You should have just used std/boost::shared_ptr and added your COW 
> logic on top.
> Your code reinvents the wheel, is quite inefficient (be it in terms of 
> memory layout, construction or assignment), and I'm not even sure this 
> is really thread-safe. Lockfree programming is tricky.
> If I remember right, boost::shared_ptr required to make some copy 
> operations atomic as well, which requires a spinlock or 128-bit CAS. 
> Surely this also applies here.

It wouldn't work to add the cow logic on top of a shared_ptr, when 
polymorphism is involved. Then an explicit clone() member function of 
the template type would be needed. The nice thing about the cow_ptr is 
that you can get this cloning facility non-intrusively.

Concerning thread-safety: Calling member functions of cow_ptr is not 
atomic. However, two cow_ptrs that point to the same object can be used 
simultaneously to modify the object. In doubt both cow_ptrs will both 
make a copies of the pointed to object. In this sense it is safe to use 
cow_ptrs in multithreaded environments. However, my cow_ptr class is not 
thread-safe in the sense that you could write to one cow_ptr from 
several threads simultaneously. Implementing this might lead to a bad 
performance penalty.