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Groups > comp.lang.c++ > #85768
| From | Keith Thompson <Keith.S.Thompson+u@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: Differences between C and C++ |
| Date | 2022-07-31 17:03 -0700 |
| Organization | None to speak of |
| Message-ID | <871qu0x21j.fsf@nosuchdomain.example.com> (permalink) |
| References | (7 earlier) <tc03ru$1ld7$1@gioia.aioe.org> <tc1t64$ert$1@gioia.aioe.org> <tc646u$gm5$1@gioia.aioe.org> <tc6bq1$1r17$1@gioia.aioe.org> <8699b402-e8e2-4e85-8de6-d67249cbeca9n@googlegroups.com> |
Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
> On Sunday, 31 July 2022 at 17:49:22 UTC+1, F.Zwarts wrote:
>> Op 31.jul..2022 om 16:39 schreef Juha Nieminen:
>> > Manfred <non...@add.invalid> wrote:
>> >> That, and below, is generally true for a generic dynamic container.
>> >> However, std::string is not a generic container. It is a very specific
>> >> and very optimized container of chars.
>> >> So, if you use a decent implementation of std the performance of
>> >> std::string and char* is usually pretty close.
>> >
>> > I don't know if you are referring to it, but many people seem to think that
>> > "short string optimization" makes std::string pretty much as efficient
>> > as an array of char (for strings that are short enough).
>> >
>> > They fail to take into consideration that short string optimization
>> > requires conditionals in almost all member functons that access the
>> > string data. Conditionals are not free.
>> I wonder if that is true. The string object has a pointer to its buffer
>> and a length. I assume that if the short string optimization is used,
>> this pointer points to the internal buffer. Only member functions that
>> want to increase the size of the buffer need those conditionals. I
>> wonder whether "almost all member functions" modify the buffer size.
>>
> The std::string is (in C)
> struct string
> {
> char *buff;
> size_t len;
> }
> so 16 bytes on a 64-bit machine.
That's one possible implementation.
> However in reality not all 64 bits of a pointer are wired to memory addresses.
> It's likely that the most significant bit has to be clear in a valid address.
> So we can exploit this by (in C)
> struct shortstring
> {
> bool flag; // set when the shortstring is valid
> int pad:7; // Maybe use for length or encoding or other stuff
> char data[15]; // UP to 14 characters of ASCII string data.
> };
>
> union
> {
> struct string s;
> struct shortstring ss;
> } std_string;
>
> Now string::size is implemented as
> if(s->ss.flag)
> return strlen(s->ss.data);
> else
> return s->s.len;
>
>
> The other string member functions are implemented similarly.
That won't work without extra code to ensure that the short string
optimization isn't used in all cases. The length of a std::string
is not determined by a null character. This program must print 3 :
#include <iostream>
#include <string>
int main() {
std::string s = "a";
s += '\0';
s += 'b';
std::cout << s.size() << '\n';
}
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */
Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-25 13:14 +0000
Re: Differences between C and C++ "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-07-25 16:18 +0200
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-25 18:47 +0300
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-25 19:49 +0300
Re: Differences between C and C++ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-25 19:38 +0100
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-25 08:51 -0700
Re: Differences between C and C++ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-25 17:14 +0100
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-25 16:19 +0000
Re: Differences between C and C++ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-25 19:28 +0100
Re: Differences between C and C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-25 12:21 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-26 07:54 +0000
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-26 08:00 +0000
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-26 08:09 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-26 02:42 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-26 15:29 +0000
Re: Differences between C and C++ scott@slp53.sl.home (Scott Lurndal) - 2022-07-26 16:30 +0000
Re: Differences between C and C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-26 11:59 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-27 07:52 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-27 01:56 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-27 14:51 +0000
Re: Differences between C and C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-27 11:11 -0700
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-26 06:09 +0000
Re: Differences between C and C++ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-07-27 17:33 +0100
Re: Differences between C and C++ Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-07-25 17:45 +0100
Re: Differences between C and C++ Paul N <gw7rib@aol.com> - 2022-07-27 10:09 -0700
Re: Differences between C and C++ Bo Persson <bo@bo-persson.se> - 2022-07-27 19:28 +0200
Re: Differences between C and C++ Paul N <gw7rib@aol.com> - 2022-07-27 11:43 -0700
Re: Differences between C and C++ David Brown <david.brown@hesbynett.no> - 2022-07-29 18:04 +0200
Re: Differences between C and C++ Richard Damon <Richard@Damon-Family.org> - 2022-07-29 18:47 -0400
Re: Differences between C and C++ David Brown <david.brown@hesbynett.no> - 2022-07-30 14:08 +0200
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-28 08:03 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-28 01:57 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-28 09:04 +0000
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-28 06:53 +0000
Re: Differences between C and C++ Richard Damon <Richard@Damon-Family.org> - 2022-07-28 07:33 -0400
Re: Differences between C and C++ Bo Persson <bo@bo-persson.se> - 2022-07-28 15:04 +0200
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-28 15:11 +0000
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-28 20:03 +0300
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-29 07:56 +0000
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-29 12:03 +0300
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-29 09:28 +0000
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-29 12:27 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-29 05:43 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-29 14:14 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-29 08:25 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-29 15:48 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-29 13:12 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-30 09:28 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-30 03:39 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-30 14:23 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-30 19:07 -0700
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-31 07:22 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-31 06:07 -0700
Re: Differences between C and C++ muttley@dastardlyhq.com - 2022-07-31 16:36 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-08-01 01:12 -0700
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-29 22:47 +0000
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-30 03:48 -0700
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-07-29 05:16 -0700
Re: Differences between C and C++ Manfred <noname@add.invalid> - 2022-07-30 02:14 +0200
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-07-31 14:39 +0000
Re: Differences between C and C++ "Fred. Zwarts" <F.Zwarts@KVI.nl> - 2022-07-31 18:49 +0200
Re: Differences between C and C++ Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-31 14:49 -0700
Re: Differences between C and C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-07-31 17:03 -0700
Re: Differences between C and C++ Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-08-01 00:16 -0700
Re: Differences between C and C++ Ike Naar <ike@sdf.org> - 2022-08-01 07:27 +0000
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-01 10:47 +0300
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-01 10:34 +0300
Re: Differences between C and C++ Manfred <noname@add.invalid> - 2022-08-03 01:20 +0200
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-08-03 07:59 +0000
Re: Differences between C and C++ Bo Persson <bo@bo-persson.se> - 2022-08-03 12:45 +0200
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-08-03 10:53 +0000
Re: Differences between C and C++ Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-08-03 03:58 -0700
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-03 14:49 +0300
Re: Differences between C and C++ Öö Tiib <ootiib@hot.ee> - 2022-08-03 23:31 -0700
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-08-01 11:29 +0000
Re: Differences between C and C++ Bo Persson <bo@bo-persson.se> - 2022-08-01 14:39 +0200
Re: Differences between C and C++ Juha Nieminen <nospam@thanks.invalid> - 2022-08-02 05:43 +0000
Re: Differences between C and C++ Manfred <noname@add.invalid> - 2022-08-03 01:13 +0200
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-03 09:34 +0300
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-29 09:22 +0000
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-29 13:53 +0300
Re: Differences between C and C++ Paavo Helde <eesnimi@osa.pri.ee> - 2022-07-29 14:13 +0300
Re: Differences between C and C++ Muttley@dastardlyhq.com - 2022-07-29 14:11 +0000
Re: Differences between C and C++ Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-07-28 06:16 -0700
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