Making a smart pointer which works with incomplete types

[James Kanze]

Re: Making a smart pointer which works with incomplete types

On Sep 8, 6:29 pm, Juha Nieminen <nos...@thanks. invalidwrote:

James Kanze wrote:

In this case you have to always pay the price of a per-object
deleter even if you don't use one.

And what is that price?

sizeof(T*) * amount_of_objec ts

Maybe, maybe not. On a Sun Sparc, in 32 bit mode, the price is
0. (Allocation granularity is 8 bytes, to ensure alignment of a
double. The reference count is 4 bytes. There are thus 4 bytes
leftover when it is allocated. Just the size of a T*.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Juha Nieminen]

Re: Making a smart pointer which works with incomplete types

James Kanze wrote:

Maybe, maybe not. On a Sun Sparc, in 32 bit mode, the price is
0. (Allocation granularity is 8 bytes, to ensure alignment of a
double. The reference count is 4 bytes. There are thus 4 bytes
leftover when it is allocated. Just the size of a T*.)

Question: Will shared_ptr in the new standard support user-defined
custom allocators for the smart pointer to allocate its data?

[James Kanze]

Re: Making a smart pointer which works with incomplete types

On Sep 9, 5:13 pm, Juha Nieminen <nos...@thanks. invalidwrote:

James Kanze wrote:

Maybe, maybe not. On a Sun Sparc, in 32 bit mode, the price is
0. (Allocation granularity is 8 bytes, to ensure alignment of a
double. The reference count is 4 bytes. There are thus 4 bytes
leftover when it is allocated. Just the size of a T*.)

Question: Will shared_ptr in the new standard support
user-defined custom allocators for the smart pointer to
allocate its data?

I hope not. Too much added complexity for too little benefit.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Kai-Uwe Bux]

Re: Making a smart pointer which works with incomplete types

James Kanze wrote:

On Sep 9, 5:13 pm, Juha Nieminen <nos...@thanks. invalidwrote:

>James Kanze wrote:

Maybe, maybe not. On a Sun Sparc, in 32 bit mode, the price is
0. (Allocation granularity is 8 bytes, to ensure alignment of a
double. The reference count is 4 bytes. There are thus 4 bytes
leftover when it is allocated. Just the size of a T*.)

>

>Question: Will shared_ptr in the new standard support
>user-defined custom allocators for the smart pointer to
>allocate its data?

>
I hope not. Too much added complexity for too little benefit.

The draft n2691 has an allocator for shared_ptr. Interestingly, I did not
find a get_allocator() function.


Best

Kai-Uwe Bux

[Pete Becker]

Re: Making a smart pointer which works with incomplete types

On 2008-09-10 03:30:50 -0400, James Kanze <james.kanze@gm ail.comsaid:

On Sep 9, 5:13 pm, Juha Nieminen <nos...@thanks. invalidwrote:

>James Kanze wrote:

>>Maybe, maybe not. On a Sun Sparc, in 32 bit mode, the price is
>>0. (Allocation granularity is 8 bytes, to ensure alignment of a
>>double. The reference count is 4 bytes. There are thus 4 bytes
>>leftover when it is allocated. Just the size of a T*.)

>

>Question: Will shared_ptr in the new standard support
>user-defined custom allocators for the smart pointer to
>allocate its data?

>
I hope not. Too much added complexity for too little benefit.

It's there, and the complexity in the specification isn't bad, because
the allocator isn't part of the type. There's an additional constructor
that takes an extra argument that specifies an allocator, an additional
overload of reset() that also takes an extra argument, and a new
non-member template function, allocate_shared , that takes an allocator.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Pete Becker]

Re: Making a smart pointer which works with incomplete types

On 2008-09-10 08:06:34 -0400, Kai-Uwe Bux <jkherciueh@gmx .netsaid:

>
The draft n2691 has an allocator for shared_ptr. Interestingly, I did not
find a get_allocator() function.

The type of the allocator isn't part of the type, so a get_allocator
function would have to be the same sort of abomination as get_deleter:
if you guess the type correctly, you get a pointer to the object,
otherwise a null pointer.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)