Re: [boost] [shared_ptr] Design question about make_shared

Subject: Re: [boost] [shared_ptr] Design question about make_shared
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-06-16 06:22:45

Stephan T. Lavavej wrote:
> [Fabio Fracassi]
> > speaking of which (i.e. deleter and make_shared) is there any way to get
> > the optimized layout and a custom deleter? Because if I understand
> > correctly neither make_shared nor allocate_shared can be used with
> > custom deleters?
>
> Ordinary shared_ptrs have separate objects and reference count control 
> blocks. The object, allocated by the user, needs to be disposed of - 
> that's what a custom deleter is for. Custom allocators are used to 
> allocate and deallocate the control block.
>
> With make_shared<T>(), the object isn't constructed by the user, it's 
> constructed by the shared_ptr, and it lives within the control block. 
> Therefore, there's no need for a custom deleter. allocate_shared<T>()'s 
> custom allocator performs the single dynamic memory allocation and 
> deallocation.

It can still make sense to have a custom deleter, to be used by make_shared 
in place of the explicit destructor call it issues on the embedded object. I 
don't have a specific use case in mind now though, maybe Fabio can provide 
one. Something like make_shared<int> to hold a file descriptor, with close() 
needing to be called instead of ~int.

One can implement something like that from the user side, via the equivalent 
of make_shared< pair<T,D> >, where the pair has a destructor that calls 
d_(t_), and then using the aliasing constructor on the result to get a 
shared_ptr<T>.

On this same note, allocate_shared ought to use the construct and destroy 
members of the allocator, so it also provides a similar (but not quite the 
same) customization point.