[PATCH] Optimize pointers to member function

[Richard Smith]

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:
> > > This patch optimize comparison and calling of pointers to member
> function.
> > >
> > > If we know that a given class do not have multiple inheritance, we can
> > > simplify the comparison and the calls as the pointer to this will
> never be
> > > shifted.
> > > On the Microsoft ABI (which is not implemented in clang), the size of a
> > > pointer to a member function is even reduced in that case.[1]
> >
> > 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.

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


> 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.
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