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

Subject: Re: [boost] [shared_ptr] Design question about make_shared
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2011-06-16 07:37:34

On 16/06/2011 06:43, Stephan T. Lavavej wrote:
> [Peter Dimov]
>> My initial implementation of boost::make_shared was a self-contained header
>> that didn't require any changes to shared_ptr (it would have worked with
>> std::tr1::shared_ptr as well as it worked with boost::shared_ptr). This can
>> only be done with a free function.
>
> Aha - that's an interesting bit of history!
>
>> (However, it can be argued
>> that this support - the ability to ask for a deleter of a specific type to
>> be default-constructed in place by shared_ptr - should be offered to users
>> as well, not just to make_shared.)
>
> I don't look at Boost's implementation, but this makes me curious as to whether you implement the "we know where you live" optimization. On x86, for a 4-byte Foobar, how big is your reference count control block for make_shared<Foobar>()? Mine is 16 bytes.

What's the point? It still need to be padded to 32 for good performance 
and to use DWCAS.