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


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.


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