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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
>> <>. 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.

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