Re: [review][fixed_string] End of review periodapproaching and a plea

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2019-12-04 23:45:01

On 2019-12-05 02:30, Peter Koch Larsen wrote:
> On Thu, Dec 5, 2019 at 12:07 AM Andrey Semashev via Boost
> <boost_at_[hidden]> wrote:
>>
>> C compatibility beyond zero termination of strings is non-existant. You
>> have that special convention, and that is fine, but that convention is
>> not standard and only you know and follow it. No C function would be
>> able to use that extra information without explicit support. You special
>> use case does not make an argument for designing a general utility like
>> fixed_string.
> It is not a convention. It is a question of enforcing a memory size
> and a layout.
> For embedded systems, this can be important. We have types that are
> required to be standard layout and have a alignment of 1 - something
> that we enforce programmatically.

When you need a specific memory layout, you should use a specialized 
structure or direct byte-wise memory accesses. No general purpose 
utility guarantees any particular binary representation, and neither 
should fixed_string.

> I believe that fixed_string is sufficiently specialised to also take
> embedded development into consideration.

fixed_string can be optimized for speed or space considerations, which 
may play in favor of embedded systems, but it should not be specialized 
to embedded systems, let alone to a specific memory layout.

>>   From the space standpoint, there is little difference between N and N+1
>> or even N+4 or N+8 bytes for a fixed_string<N> object. Given this, it is
>> preferable to choose a data layout that is more efficient in terms of
>> memory accesses and computation complexity on typical use.
> It is not just N + 4 or N + 8. Considering alignment restrictions it
> could be N + 7 or N + 15. This is significant and also a waste of
> cache.

If you place the size before the array, you will normally not have 
unused space between the size and the storage because the alignment of 
the storage is less than that of the size. You can only have alignment 
gap up to 3 bytes if the size alignment is lower than that of the 
character type (i.e. that means the worst case of fixed_string<N, 
char32_t> will have size of N+4, if the size is represented by 1 to 4 
bytes, and N+8, if it is 8).