Path: csiph.com!eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: C++20 concepts rocks
Date: Sun, 06 Feb 2022 02:11:31 -0800
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red floyd <no.spam.here@its.invalid> writes:

> On 2/4/2022 11:53 PM, Tim Rentsch wrote:
>
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>>
>>> Bonita Montero <Bonita.Montero@gmail.com> writes:
>>>
>>>> Am 04.02.2022 um 15:25 schrieb Muttley@dastardlyhq.com:
>>>>
>>>>> On Fri, 4 Feb 2022 13:54:27 +0100
>>>>> Bonita Montero <Bonita.Montero@gmail.com> wrote:
>>>>>
>>>>>> I've just written a small routine:
>>>>>>
>>>>>> [.. some c++ code ..]
>>>>>
>>>>> Thats nice.  Personally I'd just use printf().
>>>>
>>>> You can't do what I did with (s)printf().
>>>
>>> template<typename StringType>
>>> requires is_same_v<StringType,
>>>                     basic_string<typename StringType::value_type,
>>>                                  typename StringType::traits_type,
>>>                                  typename StringType::allocator_type>>
>>> StringType formatClockCycles(uint64_t clockCycles)
>>> {
>>>     char result[28], *ep = (&result)[1];
>>>     do {
>>>        sprintf(ep - 4, "%03lu", clockCycles % 1000);
>>>        if (ep != (&result)[1]) ep[-1] = '.';
>>>        ep -= 4;
>>>     } while (clockCycles /= 1000);
>>>     while (*ep == '0' && ep[1]) ep++;
>>>     return ep;
>>> }
>>>
>>> (the appropriate comment on the 28 is left as an exercise to the reader!)
>>
>> Most of the work can be done using only a single call to sprintf().
>> (Disclaimer:  not compiled.)
>>
>>
>>      template< typename StringType >
>>          requires
>>              is_same_v<
>>                  StringType,
>>                  basic_string<
>>                      typename StringType::value_type,
>>                      typename StringType::traits_type,
>>                      typename StringType::allocator_type
>>                  >
>>              >
>>      StringType
>>      formatClockCycles( uint64_t clockCycles ){
>>          char result[ 27 ];
>>          int n = sprintf( result, "%" PRIu64, clockCycles );
>>          char *ep = (&result)[1];
>>
>>          *--ep = 0;
>>          while(  n > 3  ){
>>              memmove( ep -= 3, &result[ n -= 3 ], 3 );
>>              *--ep = '.';
>>          }
>>          do  *--ep = result[ --n ];  while(  n > 0  );
>>
>>          return  ep;
>>      }
>
> Why the oddly unreadable initialization of ep?

Because it was used in the posting to which I was responding.  I
simply copied it from there, not wanting to confuse matters with
unnecessary changes.

> Why not just
>
>     char *ep = result + sizeof(result)?

My usual practice is a somewhat different construction, suitably
encapsulated so as not to sprinkle idiomatic phrases throughout
the main program text.