[PATCH] Optimize pointers to member function
Olivier Goffart
ogoffart at kde.org
Thu Feb 14 11:58:26 PST 2013
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.
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.
--
Olivier
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