kimgr added inline comments.
================
Comment at: include/string_view:216
@@ +215,3 @@
+ basic_string_view(const _CharT* __s)
+ : __data(__s), __size(_Traits::length(__s)) {}
+
----------------
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.
https://reviews.llvm.org/D21459
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