Re: cstring_view

From: Daniela Engert (dani_at_[hidden])
Date: 2022-05-14 08:31:20

Am 14.05.2022 um 09:45 schrieb Rainer Deyke via Boost:
> On 14.05.22 00:42, Marshall Clow via Boost wrote:
>>
>>
>>> On May 13, 2022, at 3:19 PM, Peter Dimov via Boost 
>>> <boost_at_[hidden]> wrote:
>>>
>>> Marshall Clow wrote:
>>>> On May 13, 2022, at 12:29 PM, Rainer Deyke via Boost
>>>> <boost_at_[hidden]> wrote:
>>>>> That's an issue with views in general, not just cstring_view.
>>>>>
>>>>> std::string s = "hello";
>>>>> string_view sv = s;
>>>>> assert(sv.size() == 5);
>>>>> s += "!";
>>>>> assert(sv.size() == 5); // boom
>>>>
>>>> I don’t see the problem here (and when I run the code I get no 
>>>> error - after
>>>> adding the missing ’std::').
>>>>
>>>> No assertion failure; no undefined behavior (unlike the 
>>>> cstring_view example)
>>>
>>> Only because "hello!" fits into the small buffer, I suspect. If `s` 
>>> reallocates,
>>> `sv` would be left dangling.
>>
>> Agreed.
>> But even if the string *did* reallocate, the call "assert(sv.size() 
>> == 5)” is still valid and well defined.
>
> No it's not.  sv.size() works by subtracting pointers, and it's only
> legal to subtract two pointers if they point into the same memory
> region.  Which sv.begin() and sv.end() no longer do if s reallocates.
> It's subtle, but it's definitely undefined behavior.
>
Not really. The standard (in its current draft) is silent about the 
invalidation of size() and talks only about iterators, references and 
pointers with respect to the viewed object 
[string.view.template.general]/2. On top of that, afaik all major 
implementations have agreed on and settled on the same structure layout 
as shown in the standard as exposition only. So technically, this is 
unspecified.

Ciao
 Â  Dani

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