On Aug 3, 2014, at 11:31 PM, Richard Smith <[email protected]> wrote:
> Alias templates allow the production of new forms of sizeof... for which we
> have no specified mangling. Example:
> struct A {
> template<typename...T> using N = int[sizeof...(T)];
> template<typename...A, typename B, typename...C>
> void f(N<A..., B, C...> &, B, C...);
> };
> void g(A a) { int arr[6]; a.f<int, int>(arr, 1, 2, 3, 4); }
> No implementation I have access to produces anything reasonable here:
>
> Clang currently produces a bogus RA3_i mangling for the type of the first
> parameter.
> GCC produces RAstDpT__i, that is, sizeof-type applied to pack expansion
> applied to template-parameter-0, which seems to be exposing a detail of their
> internal representation, and is in any case wrong since T_ is not enough to
> specify which packs are relevant.
> EDG rejects.
>
> Also of note: through alias templates, arbitrary template argument
> expressions can be incorporated into such a sizeof... expression (as can an
> implicit test that multiple packs are the same length), so it's not
> sufficient to merely encode a sequence of packs whose sizes should be added,
> along with an additional constant for packs of known size.
>
> Suggestion: extend the existing
>
> sZ <template-param>
> sZ <function-param>
>
> with
>
> sZ J <template-arg>* E
>
> for the case of a sizeof... that cannot be expressed as sizeof...(T). So the
> above parameter would mangle as
>
> sZ J DpT_ T0_ DpT1_ E
>
> I've implemented this, and it seems to work fine. Does this seem OK?
It looks reasonable. Any motivation for choosing "sZJ"? Could we use a
two-letter code instead? E.g., "sP <template-arg>* E"?
Daveed
_______________________________________________
cxx-abi-dev mailing list
[email protected]
http://sourcerytools.com/cgi-bin/mailman/listinfo/cxx-abi-dev
