[patch] Make vector::at() assertion message more useful (try #2)

[Daniel Krügler]

2013/9/4 Paul Pluzhnikov <ppluzhnikov@google.com>:
> Greetings,
>
> This is a followup to:
> http://gcc.gnu.org/ml/libstdc++/2013-08/msg00096.html
>
> Without this patch, the user on vector::at out of bounds sees:
>
> terminate called after throwing an instance of 'std::out_of_range'
>   what():  vector::_M_range_check
> Aborted (core dumped)
>
> With the patch:
>
> terminate called after throwing an instance of 'std::out_of_range'
>   what():  vector::_M_range_check: __n (which is 3) >= this->size() (which is 2)
> Aborted (core dumped)
>
>
> I am not at all sure the names I choose here are good ones. Suggestions
> welcome.
>
> I also shudder at the idea of repeating _M_range_check code in
> e.g. string::at(), and elsewhere. Perhaps we need a snprintf_lite, that
> only understands '%zu' and literal characters, e.g.:

I expect that this kind of index violation is a rather often occurring
pattern and should be isolated. IMO the _M_range
_check now pessimisms the normal, non-violating case. Why not simply
replacing it by something along the lines of

 _M_range_check(size_type __n) const
       {
        if (__n >= this->size())
         __throw_out_of_range(__index_out_of_range_msg(__N("vector::_M_range_check"),
__n, this->size()));
       }

where __index_out_of_range_msg is a reusable function that uses
something like snprintf_lite internally?

- Daniel