Re: [PATCH 2/2] Add gdb::string_view

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2018-03-17 19:49, Simon Marchi wrote:
> We had a few times the need for a data structure that does essentially
> what C++17's std::string_view does, which is to give an std::string-like
> interface (only the read-only operations) to an arbitrary character
> buffer.
>
> I first copied the string_view file from today's gcc master
> (b427286632d7) and adapted it (I don't think there should be any legal
> issues since the copyright should already belong to the FSF):
>
> - I removed things related to wstring_view, u16string_view and
>   u32string_view (I don't think we need them, but we can always add them
>   later).
> - I removed usages of _GLIBCXX_BEGIN_NAMESPACE_VERSION and
>   _GLIBCXX_END_NAMESPACE_VERSION.
> - I put the code in the gdb namespace.  I had to add a few "std::" in front of
>   std type usages.
> - I added a constructor that builds a string_view from an std::string,
>   so that we can pass strings to string_view parameters seamlessly.
>   Normally, that's handled by "operator __sv_type" in the std::string
>   declaration, but it only exists when building with c++17.
> - When building with >= c++17, gdb::string_view is an alias of
>   std::string_view.
>
> The result is close enough to the original file that if we ever need to
> update it, it should be easy enough to compare it with the new version
> in a diff editor and merge the new changes in.

Hmm, when building with older g++ (such as the aarch64 builders on the buildbot,
which have g++ 4.8), it trips on:

  using __idt = std::common_type_t<_Tp>;

It looks like that release of g++ didn't have std::common_type_t.  I guess it
would be possible to avoid using it, and change these:

    operator==(basic_string_view<_CharT, _Traits> __x,
               __detail::__idt<basic_string_view<_CharT, _Traits>> __y) noexcept

for

    operator==(basic_string_view<_CharT, _Traits> __x,
               basic_string_view<_CharT, _Traits> __y) noexcept

but I am not aware of what consequences it would have.

Simon