[Patch, libstdc++] Add specific error message into exceptions

[Jonathan Wakely]

On 28/08/15 11:23 -0700, Tim Shen wrote:
>On Fri, Aug 28, 2015 at 8:59 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
>> There seems to be no need to construct a std::string here, just pass a
>> const char* (see below).
>
>To be honest, I wasn't considering performance for a bit, since
>exceptions are already considered slow by me :P. But yes, we can do
>less allocations.
>
>> I wonder if we want to make this more efficient by adding a private
>> member to regex_error that would allow information to be appended to
>> the string, rather then creating a new regex_error with a new string.

In case it wasn't clear, I was suggesting to add a private member
*function* not data member.

>I can add a helper function to _Scanner to construct the exception
>object for only once. For functions that can't access this helper, use
>return value for error handling.
>
>> I suggest adding another overload that takes a const char* rather than
>> std::string. The reason is that when using the new ABI this function
>> will take a std::__cxx11::string, so calling it will allocate memory
>> for the string data, then that string is passed to the regex_error
>> constructor which has to convert it internally to an old std::string,
>> which has to allocate a second time.
>
>First, to make it clear: due to _M_get_location_string(), we need
>dynamic allocation.
>
>So is it good to have an owned raw pointer stored in runtime_error,
>pointing to a heap allocated char chunk, which will be deallocated in
>regex_error's dtor?

No, adding that pointer is an ABI change.

If you can't do it without an ABI change then you will have to lose
the _M_get_location_string() functionality. It seems non-essential
anyway.