Re: cstring_view

From: Rainer Deyke (rdeyke_at_[hidden])
Date: 2022-05-14 07:45:23

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.

-- 
Rainer Deyke (rainerd_at_[hidden])