http://d.puremagic.com/issues/show_bug.cgi?id=8545

           Summary: defined opCast disables cast(void*)this in classes
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-08-12 20:33:46 PDT ---
class Foo
{
    T opCast(T : int)() { return 1; }
    void* test() { return cast(void*)this; }
}

void main() { }

This is problematic, cast(void*)this is the trick used to get the address of 'this' object. &this is the address of the reference and can't be used for the same purpose. So defining opCast for any type ends up disabling 'cast(void*)this'.

I honestly think we should have a druntime function or some kind of compiler intrinsic to get the equivalent of 'cast(void*)this'. That cast itself looks too hacky to begin with.

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--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-08-12 20:35:06 PDT ---
Oh and the error message, not that it matters:

Error: template instance opCast!(void*) opCast!(void*) does not match template
declaration opCast(T : int)

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla@digitalmars.com
         Resolution|                            |INVALID


--- Comment #2 from Walter Bright <bugzilla@digitalmars.com> 2012-08-12 21:50:50 PDT ---
I don't think this is major, because you can use a union:

{
    static union U { Object o; void* p; }
    U u;
    u.o = this;
    return u.p;
}

In fact, because a union can be used for reinterpret casting, I don't think any new syntax or semantics are needed.

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--- Comment #3 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-08-13 07:57:58 PDT ---
(In reply to comment #2)
> I don't think this is major, because you can use a union:
> 
> {
>     static union U { Object o; void* p; }
>     U u;
>     u.o = this;
>     return u.p;
> }
> 
> In fact, because a union can be used for reinterpret casting, I don't think any new syntax or semantics are needed.

I'm speaking in terms of wrapping C++ libraries (A C++ wrapper class has an inner d_object pointer which it uses to invoke D virtual methods). You're asking me to introduce overhead for every time a new class object is instantiated.

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art.08.09@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |art.08.09@gmail.com


--- Comment #4 from art.08.09@gmail.com 2012-08-14 03:21:53 PDT ---
(In reply to comment #3)
> (In reply to comment #2)
> > I don't think this is major, because you can use a union:
> > 
> > {
> >     static union U { Object o; void* p; }
> >     U u;
> >     u.o = this;
> >     return u.p;
> > }
> > 
> > In fact, because a union can be used for reinterpret casting, I don't think any new syntax or semantics are needed.
> 
> I'm speaking in terms of wrapping C++ libraries (A C++ wrapper class has an inner d_object pointer which it uses to invoke D virtual methods). You're asking me to introduce overhead for every time a new class object is instantiated.

The compiler /should/ handle it w/o any additional runtime overhead. You could also use

    void* test() { return *cast(void**)&this; }

Which approach will be more efficient probably depends on compiler implementation; ideally both versions should be free.

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2012-08-14 08:18:35 PDT ---
> You could also use
> 
>     void* test() { return *cast(void**)&this; }
> 

Thanks! It does seem to create the exact same assembly even with no optimizations.

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