[PATCH] Optimize pointers to member function

[Olivier Goffart]

On Thursday 14 February 2013 12:20:31 Richard Smith wrote:
> On Thu, Feb 14, 2013 at 11:58 AM, Olivier Goffart <ogoffart at kde.org> wrote:
> > On Thursday 14 February 2013 11:05:28 John McCall wrote:
> > > On Feb 14, 2013, at 3:57 AM, Olivier Goffart <ogoffart at kde.org> wrote:
> > > 
> > > A pointer-to-member may point to a member of a derived class.
> > 
> > How?
> > 
> > struct A { void m(); };
> > struct B : A { void n(); };
> > int main() { void (A::*t)() = &B::n; }
> > 
> > error: cannot initialize a variable of type 'void (A::*)()' with an rvalue
> > of type 'void (B::*)()': different classes ('A' vs 'B')
> > 
> > C++11[4.11.2]
> > 
> >    A prvalue of type “pointer to member of B of type cv T”, where B is a
> >    class type, can be converted to a prvalue of type “pointer to member of
> >    D of type  cv T”, where D is a derived class (Clause 10) of B. [...]
> > 
> > You can convert from a base to a derived, but not the other way around.
> 
> [expr.static.cast]p12.

There I read: 

   [...] If class B contains the original member, or is a base or derived
   class of the class containing the original member, the resulting pointer to
   member points to the original member. Otherwise, the result of the cast is
   undefined.

> int main() { void (A::*t)() = (void(A::*)())&B::n; }

So that cast is undefined.

Even then, the patch don't break that cast,  what will be stop working is if 
is one does the call. But that's undefind behaviour.

And [expr.reinterpret.cast]p10 is also still working. (converting back to the 
original type)

> > Notice that since Microsoft changes the size of the pointer depending on
> > that fact, it means it cannot be possible to point to a member of a
> > derived class.
> 
> No, it means Microsoft's implementation is non-conforming. See MS compiler
> warning C4407.

I beleive that warning just warns that this is an undefined behaviour.

-- 
Olivier