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: Pyry Jahkola (pyry.jahkola_at_[hidden])
Date: 2013-02-10 05:27:56

On 2013-02-09 18:47:59 +0000, Andreas Pfaffenbichler said:

> a heavy argument for a 'COW' pattern is made here:
> http://cppnow.org/session/value-semantics-and-concepts-based-polymorphism
> ( 'copy_on_write' class template included).

I was going to reply about the same, but here's the whole class with 
documentation:

http://stlab.adobe.com/classadobe_1_1version__1_1_1copy__on__write.html

> maybe it could be interesting to compare your 'cow_ptr' to this
> 'copy_on_write'.

The biggest difference and, in my opinion, the most important advantage 
in favour of adobe::copy_on_write is that the copying is explicit; 
given a

    copy_on_write<T> x;

you can only use

    x->const_member();  // no refcount overhead even if x is non-const

to access const members of T. Whenever you need to modify the instance, 
you'll need to explicitly tell copy_on_write about it by calling 
write():

    T & r = x.write();  // write() performs the copy if needed
    r.mutating_member();

or, matching the use case of cow::cow_ptr<T>::apply:

    f(x.write());

This interface maps very nicely to dealing with value types: as long as 
you're treating everything as a value (i.e. read-only), there aren't 
any hidden costs of making a copy either. And as soon as you need to 
make changes to the data, the overhead of the internal copy will be 
visible in the code responsible of the mutations as well. I guess 
that's enough to clear most concerns about the thread-safety issues too.

-- 
Pyry Jahkola
pyry.jahkola_at_[hidden]
https://twitter.com/pyrtsa