mclow.lists added inline comments.
================
Comment at: include/string_view:216
@@ +215,3 @@
+ basic_string_view(const _CharT* __s)
+ : __data(__s), __size(_Traits::length(__s)) {}
+
----------------
kimgr wrote:
> mclow.lists wrote:
> > mclow.lists wrote:
> > > kimgr wrote:
> > > > mclow.lists wrote:
> > > > > kimgr wrote:
> > > > > > I'm working from the paper at
> > > > > > https://isocpp.org/files/papers/N3762.html, and I find it a little
> > > > > > sketchy on the policy for nullptrs.
> > > > > >
> > > > > > Since the ctor above accepts nullptr as long as the length is zero,
> > > > > > would it make sense to do that here too? That is, only call
> > > > > > _Traits::length for non-nullptr __s args?
> > > > > Reading from N4600: Requires: `[str, str + traits::length(str))` is a
> > > > > valid range.
> > > > >
> > > > > So, no - passing `nullptr` here is undefined.
> > > > >
> > > > OK, that feels more principled to me, anyway.
> > > >
> > > > But the `(const char*, size_t)` constructor has the same requirement
> > > > and it *does* accept `nullptr`, provided the length is zero. I saw you
> > > > had to get rid of the assertion, but I feel like the constructor should
> > > > probably not silently accept `nullptr` for zero-sized strings. Or do
> > > > you know if that's intentional? Many StringRef/StringPiece
> > > > implementations seem to have the same behavior.
> > > It is absolutely intentional. `[nullptr, nullptr+0)` is a perfectly fine
> > > half-open range. It contains no characters.
> > >
> > > However, the ctor that takes just a pointer has to calculate the length
> > > .. by dereferencing the pointer.
> > >
> > > I had to get rid of the assertion because one of the bots (gcc 4.9) has a
> > > bug about constexpr ctors in c++11 mode. Even though the assertion was
> > > `#ifdef`ed on C++ > 11, the compiler complained about it. I'll be
> > > putting the assertion back as soon as I can figure out how to keep gcc
> > > from complaining.
> > This was discussed (at length) in LEWG during the design of `string_view`.
> Ah, got it, thanks! It opens up for cases where `data()` for an empty
> `string_view` can sometimes return `""` and sometimes `nullptr`, but I guess
> that problem extends beyond `string_view`'s responsibilities.
>
> Thanks for the thorough explanation.
I think you're laboring under a misapprehension here.
An empty `string_view` points to *no characters*, not an empty null-terminated
string.
Treating the pointer that you get back from `string_view::data` as a
null-terminated string will lead to all sorts of problems. This is explicitly
called out in [string.view.access]/19:
> Note: Unlike basic_string::data() and string literals, data() may return a
> pointer to a buffer that is not null-terminated. Therefore it is typically a
> mistake to pass data() to a routine that takes just a const charT* and
> expects a null-terminated string.
https://reviews.llvm.org/D21459
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
