Re: Segments

David Brown <david.brown@hesbynett.no> writes:
[...]

There are a couple of ways you can do "small string optimisation".
One would be to have a structure something like this :
>
struct String1 {
size_t capacity;
char * data;
char small_string[16];
}
>
Then "data" would point to "small_string" for a capacity of 16, and if
that's not enough, use malloc to allocate more space.
>
>
An alternative would be to have something like this (I'm being
/really/ sloppy with alignments, rules for unions, and so on - this is
illustrative only, not real code!) :
>
struct String2 {
bool is_small;
union {
char small_string[31];
struct {
size_t capacity;
char * data;
}
}
}

[...]

There are, of course, many other ways to make string types (such as
supporting copy-on-write), but I suspect that Mitch is thinking of
style String2 while Keith is thinking of style String1.

I wasn't particularly thinking of either.  I was reporting results
I got from experimenting with the std::string type in the GNU
implementation I use on Ubuntu.  Dumping the raw data showed string
data in the last 16 bytes of the 32-byte std::string object when
the length is no more than 15.  For longer strings, I suspect that
16-byte area is used for other purposes, so it's probably similar
to String2, but I haven't examined the source code.

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */