Re: [PATCH][_GLIBCXX_DEBUG] Add basic_string::starts_with/ends_with checks

[Jonathan Wakely <jwakely@redhat.com>]

On Sun, 14 Aug 2022 at 16:34, François Dumont via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> I think we can add those checks.
>
> Note that I wonder if it was needed as in basic_string_view I see usages
> of __attribute__((__nonnull__)). But running the test I saw no impact
> even after I try to apply this attribute to the starts_with/ends_with
> methods themselves.

That should cause warnings, and does when I try it.

As you say, the relevant string_view constructor already has that anyway:

      __attribute__((__nonnull__)) constexpr
      basic_string_view(const _CharT* __str) noexcept

And so does string_view::find. The problem is that those only help if
the compiler inlines the calls to those functions and so can propagate
the null value all the way down to a function with the attribute.
Adding the attribute to the relevant starts_with, ends_with and
contains functions makes the diagnostics more likely to be emitted
without optimization.

>
> Also note that several checks like the ones I am adding here are XFAILS
> when using 'make check' because of the segfault rather than on a proper
> debug checks. Would you prefer to add dg-require-debug-mode to those ?
>
>      libstdc++: [_GLIBCXX_DEBUG] Add basic_string::starts_with/ends_with
> checks
>
>      Add simple checks on C string parameters which should not be null.
>
>      Review null string checks to show:
>      _String != nullptr
>
>      rather than:
>      _String != 0

I don't really like the extra complexity in the macros, but this does
seem like a nice improvement for what users see.

We could use __null for C++98, which is a compiler keyword that
expands to a null pointer constant, but I'm not sure if that would be
clear to all users or not. Maybe 0 is better.