Subject: Re: [boost] [optional] memory use for optional refs and ptrs
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-10-11 07:16:27
On 10/07/10 07:31, Rutger ter Borg wrote:
> On 2010-10-07 13:37, Larry Evans wrote:
>
>> [snip]
>>
>> A more complete facility use is here:
>>
>>
>> https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates/libs/composite_storage/sandbox/layout_all.cpp
>>
[snip]
>> which shows the padding after the char is 3 in order
>> to make the alignment of the int at 4.
>>
>> HTH.
>>
>> -Larry
>
> Thanks,
>
You're welcome.
The reusable part of the code has been refactored into:
http://svn.boost.org/svn/boost/sandbox/variadic_templates
/boost/composite_storage/utility
/show_layout_all_of_aligned.hpp
so that it on different struct's rather than just the struct's
in the current example driver.
The output produced on the example struct's in:
https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates
/libs/composite_storage/sandbox/layout_all.1.cpp
is now in tabular format:
A_type
===========
***components***
----------------
num size align sum_size
1 4 4 4
2 8 8 12
3 8 8 20
4 8 8 28
***composite***
---------------
num size align pad sum_pad offset
5 32 8 0 0 _
4 32 8 0 0 24
3 24 8 0 0 16
2 16 8 4 4 8
1 4 4 0 4 0
0 0 1 _ _ _
A_bools_type
===========
***components***
----------------
num size align sum_size
1 4 4 4
2 8 8 12
3 8 8 20
4 8 8 28
5 1 1 29
6 1 1 30
7 1 1 31
8 1 1 32
***composite***
---------------
num size align pad sum_pad offset
9 40 8 4 4 _
8 36 8 0 4 35
7 35 8 0 4 34
6 34 8 0 4 33
5 33 8 0 4 32
4 32 8 0 4 24
3 24 8 0 4 16
2 16 8 4 8 8
1 4 4 0 8 0
0 0 1 _ _ _
bools_A_type
===========
***components***
----------------
num size align sum_size
1 1 1 1
2 1 1 2
3 1 1 3
4 1 1 4
5 4 4 8
6 8 8 16
7 8 8 24
8 8 8 32
***composite***
---------------
num size align pad sum_pad offset
9 32 8 0 0 _
8 32 8 0 0 24
7 24 8 0 0 16
6 16 8 0 0 8
5 8 4 0 0 4
4 4 1 0 0 3
3 3 1 0 0 2
2 2 1 0 0 1
1 1 1 0 0 0
0 0 1 _ _ _
which shows the padding between individual
fields and the sum of the padding. This shows
why putting the bools first (bools_A_type)
saves space. As mentioned before the 4 bools
are placed in the padding present in the
padding required by A_type; hence,
sizeof(A_type) == sizeof(bools_A_type).
Hope the tabular output makes it easier
to see the reason for space usage.
-Larry