On Aug 4, 2014, at 10:45 AM, Richard Smith <[email protected]> wrote:
> On 4 August 2014 06:50, David Vandevoorde <[email protected]> wrote:
>
> 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"?
>
> I really wanted to replace the existing rules with sZ <template-arg>... but
> that's backwards-incompatible for function parameter packs and non-type
> template parameter packs.
>
> Could we use a two-letter code instead? E.g., "sP <template-arg>* E"?
>
> Sure, works for me.
>
> (And for clarity:
>
> template<typename ...T> using N = int[sizeof...(T)];
> template<typename ...T> void f(N<T...> &);
>
> ... should use the sZ mangling, not the sP mangling, since by 14.5.7/2, you
> can redeclare this template as 'f(int (&)[sizeof...(T)])’.)
This looks fine to me. I’ll commit this in a week if there isn’t any further
commentary.
John.
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