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

Subject: Re: [boost] [smart ptr] Any interest in copy-on-write pointer forC++11?
From: Ralph Tandetzky (ralph.tandetzky_at_[hidden])
Date: 2013-02-11 15:01:46

> That interface might not prove very convenient in practice though. 
> Although you could, I think, build apply/APPLY upon it.
>
> read_ptr<T> pr;
>
> if( write_ptr<T> pw = pr.write() )
> {
>    pw->mutate();
>    pr = std::move( pw );
> }
>
> I don't, however, particularly like the semantics of the case in which 
> pr is already unique. In principle it shouldn't matter because we're 
> writing pr from this thread, so nobody ought to be reading it, but 
> still, it doesn't feel right.
>

Nice idea though. write_ptr<T> would behave like value_ptr<T> and 
read_ptr<T> would behave like shared_ptr<T const> with the additional 
ability to return write_ptrs and being constructible from an rref to 
write_ptr. The function apply can be implemented as a non-member function:
         template <typename F>
         void apply( read_ptr<T> & pr, F f )
         {
             write_ptr<T> pw = pr.write();
             assert( pw );
             f( pw.get() );
             pr = std::move( pw );
         }
The macro APPLY could be implemented similar to the way I implemented it 
for cow_ptr. Once you have apply() and/or APPLY the interface isn't so 
inconvenient anymore. write_ptr might as well just be a value_ptr. 
Possibly value_ptr<T> and read_ptr<T> need to be friends of each other.

The question for this design is: Is it worth it? I guess I need to think 
about that a little more in order to come to a conclusion.

Thanks for the idea.
Ralph