Re: [Boost-users] [review][fixed_string] FixedString review starts today

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2019-11-29 08:53:42

On 2019-11-29 11:12, Peter Dimov via Boost wrote:
>> and yes, you can write code that will be correct if op+= doesn't check.
> 
> Maybe you can, but I apparently can't. This is what I just wrote:
> 
> void my_append( fixed_string<512> & s, std::string_view s1, 
> std::string_view s2 )
> {
>  Â Â  if( s.size() + s1.size() + s2.size() > s.max_size() ) throw 
> std::length_error( "" );
> 
>  Â Â  s += s1;
>  Â Â  s += s2;
> }
> 
> Is this correct? (Spoiler: no.)

I think, appending N strings still requires N tests, unless you know 
that the string sizes combined don't overflow.

   size_t size_left = s.max_size() - s.size();
   if (s1.size() > size_left) throw_length_error();
   size_left -= s1.size();
   if (s2.size() > size_left) throw_length_error();

Or use unsigned __int128 to calculate the combined size. Alas, it's not 
universally available.