Re: [OpenMethod] review starts on 28th of April

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2025-05-12 23:14:08

AMDG

On 5/12/25 3:10 PM, Jean-Louis Leroy via Boost wrote:
>> Can we strip off the virtual_?
>>
>> struct NAME ## _overrider<...> {
>>     template<typename T>
>>     using virtual_ = T;
>>     ...
>> };
> 
> You mean allow virtual_ in the overrider's parameters? The macro gets:
>      meet, (virtual_<Dog&> dog, virtual_<Cat&> cat)
> 
> ...and it changes into:
>      meet(Dog& dog, Cat& cat)
> 
> I don't see a way of doing this while carrying the parameter's names.
> 

The above would make virtual_ into an alias that just expands to its 
argument, but this only works when virtual_ is used unqualified.

>> <snip>
> 
> It took me a while to see how this could work, and now it's taking me a while to
> see why it doesn't want to - see https://godbolt.org/z/16nensohc. Probably
> related to your "order of expansion" comment?
> 
> It is appealing but it has problems too. Any comma in the parameter's types will
> break it.

Not necessarily. You snipped the trick I proposed to deal with that.

> YOMM2 uses Boost.PP loops over macro arguments to build the forwarding
> function, so whenever there is a comma in the parameter types, you have to use
> tricks like BOOST_IDENTITY_TYPE. For OpenMethod, I moved as much macro-magic to
> TMP as I could.
> 

#define HAS_VIRTUAL_TESTvirtual ~,~
#define HAS_VIRTUAL_II(a, b, r, ...) r
#define HAS_VIRTUAL_I(...) HAS_VIRTUAL_II(__VA_ARGS__)
#define HAS_VIRTUAL(arg) HAS_VIRTUAL_I(HAS_VIRTUAL_TEST ## arg, 1, 0, ~)
#define REMOVE_VIRTUALvirtual
#define REMOVE_VIRTUAL(arg) REMOVE_VIRTUAL ## arg

#define IDENTITY(arg) arg
#define MARK_VIRTUAL(arg) boost::openmethod::detail::virtual_next, 
REMOVE_VIRTUAL ## arg

#define REMOVE_IF_VIRTUAL(arg) \
  BOOST_PP_IIF(HAS_VIRTUAL(arg), REMOVE_VIRTUAL, IDENTITY)(arg)

#define MARK_IF_VIRTUAL(arg) \
   BOOST_PP_IIF(HAS_VIRTUAL(arg), MARK_VIRTUAL, IDENTITY)(arg)

MARK_IF_VIRTUAL(virtual test)
REMOVE_IF_VIRTUAL(virtual test)
MARK_IF_VIRTUAL(nonvirtual)
REMOVE_IF_VIRTUAL(nonvirtual)

With this
   (virtual B, int, virtual C), void
becomes
   void(boost::openmethod::detail::virtual_next, B, int, 
boost::openmethod::detail::virtual_next, C)
which can be turned into void(virtual_<B>, int, virtual_<C>) by TMP. In 
the function definition, we simply remove the virtual, and everything 
else, including the argument name is preserved.

The main problem is that it blows up when a comma is followed by 
something that can't be token pasted:

(::global, templ<I, (X + Y)>)

In Christ,
Steven Watanabe