Eric Lemings wrote:
[...]
...
Without the cast, the argument type passed to the internal ctor is
still `std::tuple<_TypesT...>' and, because of the template ctors,
the compiler would therefore bind to the templated copy ctor wisely
deeming this ctor a better "fit".
Would this be prevented by not deriving tuple from __rw_tuple (or
not deriving it directly)? FWIW, it seems to me that in order to
implement the space optimization we discussed this morning, we
might need to make some changes in this area anyway. We should
keep the cast issue in mind while designing a solution.
I'll have to look into this further but I'm almost certain the changes
required to implement this optimization would make the code even more
complicated and -- if the solution is what I think it may be -- might
even require another base class and possibly more casts.
FWIW, in case you haven't seen the tuple mock up posted by Howard
to [EMAIL PROTECTED], read c++std-lib-21810. Let me quote the
last paragraph from his post:
Btw, this prototype does the empty member optimization. Last
time I checked with the LWG, this was to be allowed but not
required. I hoping we still feel that way. As far as I know,
the current WP text implies this by not saying otherwise, and
by not saying too much about the layout. It is my hope that
ultimately clients will be able to portably depend on this
characteristic:
struct Empty {};
static_assert(sizeof(tuple<int, Empty>) == sizeof(int),
"Please implement empty member optimization in tuple");
static_assert(sizeof(tuple<int, Empty, int>) == 2*sizeof(int),
"Please implement empty member optimization in tuple");
