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

On 2019-12-04 19:05, Peter Dimov via Boost wrote:

Phil Endecott wrote:

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Peter Dimov wrote:

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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.