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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: Differences between C and C++
Date: Sun, 31 Jul 2022 17:03:52 -0700
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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 */