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

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2019-12-04 18:10:41

On 2019-12-04 19:05, Peter Dimov via Boost wrote:
> Phil Endecott wrote:
>> Peter Dimov <pdimov_at_[hidden]> wrote:
> 
>> > Storing the size (as capacity - size) in the last char for N < 256 
>> will > have more impact, but I'm not sure that it too is worth the 
>> added > complexity.
>>
>> Why the last char, rather than always having the size (of whatever 
>> appropriate type) first?  Is the idea that this makes data() and 
>> c_str() essentially no-ops?
> 
> The idea here is that you win one byte by reusing the last byte of the 
> storage as the size, overlapping it with the null terminator in the 
> size() == N case (because capacity - size becomes 0).

I'm not sure this would actually be beneficial in terms if performance. 
Ignoring the fact that size() becomes more expensive, and this is a 
relatively often used function, you also have to access the tail of the 
storage, which is likely on a different cache line than the beginning of 
the string. It is more likely that the user will want to process the 
string in the forward direction, possibly not until the end (think 
comparison operators, copy/assignment, for instance). If the string is 
not close to full capacity, you would only fetch the tail cache line to 
get the string size.

It is for this reason placing any auxiliary members like size is 
preferable before the storage array. Of course, if you prefer memory 
size over speed, placing size in the last byte is preferable.