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List: cfe-commits
Subject: Re: [PATCH] Optimize pointers to member function
From: Richard Smith <richard () metafoo ! co ! uk>
Date: 2013-02-14 20:20:31
Message-ID: CAOfiQqmdZ=F=UxoNBY7ohWD3_VKPr4EB7X+FrFEYZHLgs_q90Q () mail ! gmail ! com
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On Thu, Feb 14, 2013 at 11:58 AM, Olivier Goffart <ogoffart@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@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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On Thu, Feb 14, 2013 at 11:58 AM, Olivier Goffart <span dir="ltr"><<a \
href="mailto:ogoffart@kde.org" target="_blank">ogoffart@kde.org</a>></span> \
wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 \
0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div class="im">On Thursday 14 \
February 2013 11:05:28 John McCall wrote:<br> > On Feb 14, 2013, at 3:57 AM, \
Olivier Goffart <<a href="mailto:ogoffart@kde.org">ogoffart@kde.org</a>> \
wrote:<br> > > This patch optimize comparison and calling of pointers to member \
function.<br> > ><br>
> > If we know that a given class do not have multiple inheritance, we can<br>
> > simplify the comparison and the calls as the pointer to this will never \
be<br> > > shifted.<br>
> > On the Microsoft ABI (which is not implemented in clang), the size of a<br>
> > pointer to a member function is even reduced in that case.[1]<br>
><br>
> A pointer-to-member may point to a member of a derived class.<br>
<br>
</div>How?<br>
<br>
struct A { void m(); };<br>
struct B : A { void n(); };<br>
int main() { void (A::*t)() = &B::n; }<br>
<br>
error: cannot initialize a variable of type 'void (A::*)()' with an rvalue \
of<br> type 'void (B::*)()': different classes ('A' vs \
'B')<br> <br>
<br>
C++11[4.11.2]<br>
A prvalue of type “pointer to member of B of type cv T”, where B is a class<br>
type, can be converted to a prvalue of type “pointer to member of D of type<br>
cv T”, where D is a derived class (Clause 10) of B. [...]<br>
<br>
You can convert from a base to a derived, but not the other way \
around.<br></blockquote><div><br></div><div>[expr.static.cast]p12.</div><div><br></div><div>int \
main() { void (A::*t)() = (void(A::*)())&B::n; }</div><div> </div><blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex"> Notice that since Microsoft changes the size of the pointer \
depending on that<br> fact, it means it cannot be possible to point to a member of a \
derived class.</blockquote><div><br></div><div>No, it means Microsoft's \
implementation is non-conforming. See MS compiler warning C4407.</div></div>
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