[cfe-dev] [clang] declspec(property...) advice

Eric Niebler eric at boostpro.com
Sun Feb 6 08:40:39 PST 2011

On 2/4/2011 4:00 PM, Eric Niebler wrote:
> (resurrecting this old thread....)
> On 12/9/2010 5:16 AM, John McCall wrote:
>> I'm a little worried about adopting our current representation for a
>> C++-oriented feature.  Presumably we'd need to allow filling in default
>> arguments, etc., plus a wide range of stuff that we technically have to
>> support with ObjC++ properties but actually haven't gotten around to
>> yet, like what happens when we're doing a compound assignment on an
>> implicit property reference and the getter and setter have really
>> different types.  Really I think this is an argument for improving the
>> representation of all property expressions.
> Indeed.
>> My intuition here is to just rename OK_ObjCProperty to OK_Property
>> instead of differentiating the two cases.  
> Yes.
>> It can mean "this is an
>> entity which can be both read and written to, but requires special code
>> to do so".  The existing hook in Sema for loads
>> (ConvertPropertyForRValue) can easily be modified to handle the MS
>> properties, and we can make BuildBinOp check for OK_Property LHS's and
>> just immediately defer to some better hook.
> And BuildUnaryOp for pre- and post-increment/decrement.
> Doug, I've found a compelling reason why at least setter methods cannot
> be resolved until the point of use. Consider:
>   #include <cstdio>
>   struct T {};
>   struct U {};
>   struct S {
>     void setI( T ) { std::printf("T\n"); }
>     void setI( U ) { std::printf("U\n"); }
>     __declspec(property(put=setI)) int I;
>   };
>   int main() {
>     S s;
>     s.I = T();
>     s.I = U();
>   }
> This prints:
>   T
>   U
> Nasty, eh? I don't think the existing property stuff can handle this at
> all, so like John, I feel this needs a major reworking.
> Any and all suggestions welcome. This has become my #1 priority, and I
> want to make forward progress quickly, but I want to do it right.

I'd like to share my current thinking on the property issue. I encourage
any and all interested parties to jump in.

Above, John McCall suggests modifying BuildBinOp to Do Something. I
think that something is to programatically rewrite the AST into the
appropriate getter and setter calls. Ditto for BuildUnaryOp. So for
instance, for this code:


when BuildUnaryOp gets passed a PropertyRefExpr and the
pre-increment op code, it builds the AST for this:

  obj.setProperty(obj.getPropery() + 1)

Well, actually, it /should/ return the AST for the unary op as usual,
but hidden away inside it somewhere should be the AST for the above
rewrite. Why both? The AST should accurately reflect the fact that the
user wrote it as "++obj.property". However, creating the rewritten AST
nodes has the effect of type-checking the expression, AND the rewritten
AST can be used for codegen later (as long as it's accessible somehow).

I've been looking into how to generate those AST nodes. First,
obj.setProperty gets parsed into a MemberExpr, then, the paren and the
arguments are parsed, and the whole thing gets passed to
BuildCallToMemberFunction. I figure if we do all of that in the right
places and it type checks, we're golden. Stuff the result into the AST
somewhere reasonable and that should do it.

So that's my current thinking. This, however, is a radical departure
from how properties are currently handled in clang, and will require
some pretty invasive surgery.


Eric Niebler
BoostPro Computing

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