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List: cfe-commits
Subject: Re: [PATCH] Optimize pointers to member function
From: John McCall <rjmccall () apple ! com>
Date: 2013-02-14 21:29:29
Message-ID: E3C46974-B0C9-4A17-A40E-A7907C976D0E () apple ! com
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On Feb 14, 2013, at 12:20 PM, Richard Smith <richard@metafoo.co.uk> wrote:
> 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; }
Right. You can also reinterpret_cast a pointer to member to a completely
unrelated pointer to member type, as long as they agree about function vs.
non-function. It's not undefined behavior until you try to use a pointer to
member on an object whose dynamic type does not have that member.
> 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.
Precisely.
John.
[Attachment #5 (text/html)]
<html><head><meta http-equiv="Content-Type" content="text/html \
charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: \
space; -webkit-line-break: after-white-space; "><div><div>On Feb 14, 2013, at 12:20 \
PM, Richard Smith <<a \
href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>> \
wrote:</div><blockquote type="cite">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></blockquote><div><br></div>Right. You can also reinterpret_cast a \
pointer to member to a completely</div><div>unrelated pointer to member type, as \
long as they agree about function vs.</div><div>non-function. It's not \
undefined behavior until you try to use a pointer to</div><div>member on an object \
whose dynamic type does not have that member.</div><div><br></div><div><blockquote \
type="cite"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: \
0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); \
border-left-style: solid; padding-left: 1ex; position: static; z-index: auto; "> \
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> \
</blockquote></div><br><div>Precisely.</div><div><br></div><div>John.</div></body></html>
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