Re: [GSoC] __enable_shared_from_this_helper

[Jonathan Wakely <jwakely at redhat dot com>]

On 28/04/15 16:48 -0400, Fan You wrote:
> Hi,
>
> 1. I feel confused about the function
> __enable_shared_from_this_helper(); in shared_ptr_base.h.
>
> It's defined as (on line 853) and overloaded to 2 extra version which
> accept enable_shared_from_this*:
>
> > template<_Lock_policy _Lp>
> >   inline void
> >   __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...) noexcept
> >   { }
>
>
> Then it get called in every __shared_ptr_base ctors like this,
>
> > __enable_shared_from_this_helper(_M_refcount, __p, __p);
>
>
> According to my knowledge, __enable_shared_from_this construct a
> weak_ptr in order to safely instantiate a new shared_ptr use only this
> pointer. So, does __enable_shared_from_this_helper has no effect until
> user trying to do something like this?
>
> > class Test : public ___enable_shared_from_this { };
> >
> > shared_ptr<Test> (new Test());

Correct.

It ensures that when an object is owned by a shared_ptr, the weak_ptr
member shares ownership with that shared_ptr.

> 2. Also, I am little bit unsure about what _Sp_counted_ptr_inplace does.
>
> It seems to me that when I call allocate_shared<>(); it construct a
> single _Sp_counted_ptr_inplace object.
> Then, _Sp_counted_ptr_inplace is taking charge of allocating the
> needed memory for shared_ptr by using the given allocator.
>
> Does it means, if I want to make make_shared to support array, what I
> need to do is change the constructor of _Sp_counted_ptr_inplace to
> support allocate size?

I haven't thought about it in detail, but I think you'll need a new
type for make_shared<T[]>.