https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91788
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2019-09-23
Ever confirmed|0 |1
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #0)
> IIUC, the whole +1-1 is here so that for a valueless variant, index_type(-1)
> becomes size_t(-1).
You do understand correctly :-)
> I think there are cases where we could do better. For
> instance, for a never valueless type, we could just return _M_index. If
> there are fewer than 128 alternatives, we could use a sign extension:
> "return (signed char)_M_index;".
I think we can do that more generically. Does this look right?
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1518,7 +1518,14 @@ namespace __variant
{ return !this->_M_valid(); }
constexpr size_t index() const noexcept
- { return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; }
+ {
+ using __index_type = typename _Base::__index_type;
+ if constexpr (__detail::__variant::__never_valueless<_Types...>())
+ return this->_M_index;
+ else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
+ return make_signed_t<__index_type>(this->_M_index);
+ return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1;
+ }
void
swap(variant& __rhs)
(In reply to Marc Glisse from comment #1)
> Internally, it may also be possible to avoid calling index() so often and
> work with the raw _M_index more often.
IIRC I considered that, but (at the time) decided to play it safe rather than
spend more time analysing what could be done.
I think we could replace checks like this in get<_Np>:
if (__v.index() == _Np)
with:
static_assert(index_type(_Np) == _Np && _Np != index_type(-1));
if (__v._M_index == _Np)
And in many (most?) cases the static assert is unnecessary because we already
check something like:
static_assert(_Np < sizeof...(_Types),
"The index must be in [0, number of alternatives)");
Anywhere we test equality of index() with a value that is known to be in range,
it's OK to use index_type(-1) directly instead of extending it to size_t(-1).
(The downside would be that many more functions might need to be friends to
access _M_index).
The relational operators need a bit more thought and I don't think we can
change the calls to index() in __do_visit without breaking the implied contract
with the _M_access functions in the "vtable".
