Re: [boost] [smart_ptr] make_shared usage

From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2008-07-28 14:41:17

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On Monday 28 July 2008 10:29 am, Daniel Frey wrote:
> On Mon, 2008-07-28 at 08:52 -0400, Frank Mori Hess wrote:
> > I don't think the zero-argument call to make_shared resolves to the
> > variadic template, see the make_shared.hpp file.
>
> I've seen that, but:
>
> a) I don't understand what the "new T()" vs. "new T" issue is - and
> there seems to be nothing in the code which explains that. It does use
> new T(), which should also work for new T( Args... ) if Args is a
> zero-sized parameter pack, shouldn't it?
>
> b) Looking at N2352 and N2691, there should be only one make_shared.
> Boost vs. GCC already shows that looking at a concrete implementation is
> not enough. Question is, what is/should be guaranteed by the standard.
> If nothing like this is guaranteed (probably because it's simply too
> late?), I would end up with non-portable code. Even then I would be
> interested in whether or not you, Peter, etc. think if something like
> that should be at least targeted for TR2...

I'm just saying the friend declaration might need to be simply

template< typename A >
friend std::shared_ptr< A > std::make_shared();

due to the overload for zero arguments in make_shared.hpp.  I should mention I 
didn't notice before that you were actually using std:: instead of boost:: 
and I'm not actually familiar with the gcc 4.3 implementations.

>
> d) Besides the use case above, what do you think about using this
> pattern to allow shared_from_this to be called in the ctor? Is this
> possible? Would it be useful?
>

Do you mean something like a more generic version of deconstruct_ptr:

http://www.comedi.org/projects/thread_safe_signals/libs/thread_safe_signals/doc/html/boost/deconstruct_ptr_id2684660.html

except it would have the benefits provided by make_shared, and allow a class 
to force its use via private constructor/friendship?  It seems like it could 
be useful.
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