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cxx-vtable-ex.html
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<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="GENERATOR" CONTENT="Mozilla/4.05 [en] (WinNT; U) [Netscape]">
<TITLE>C++ Vtable Example</TITLE>
<LINK rel=stylesheet href=small-table.css type="text/css">
</HEAD>
<BODY>
<HR>
<P><B><I><FONT SIZE=+3>C++ Vtable Example</FONT></I></B>
<P><I>Revised 10 September 1999</I>
<P>
<HR>
<P>[990910 IBM -- Brian] Added more examples,
split out the two kinds of adjustments in Table 1a,
and added a summary of the component counts for the two approaches.
<P>
<HR>
<TABLE BORDER=2 CELLPADDING=4 >
<CAPTION>Table 1a: Example Code and Call Semantics </CAPTION>
<TR>
<TH>Declarations </TH>
<TH>Call </TH>
<TH>Callee </TH>
<TH>Call-site
<BR>Adjustment </TH>
<TD><B>Thunk/Entry-point</B>
<CENTER><B>Adjustment</B></CENTER>
</TD>
</TR>
<TR>
<TD ROWSPAN="3">
<PRE><TT>struct A {
virtual void f ();
virtual void g ();
virtual void h ();
int ia;
};
A *pa;</TT></PRE>
</TD>
<TD><TT>pa->f()</TT></TD>
<TD>A::f() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa->g()</TT></TD>
<TD>A::g() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD>pa->h()</TD>
<TD>A::h()</TD>
<TD>none</TD>
<TD>none</TD>
</TR>
<TR>
<TD ROWSPAN="8">
<PRE><TT>struct B: public virtual A {
void f ();
void h ();
int ib;
};
B *pb;
A *pa_in_b = pb;</TT></PRE>
</TD>
<TD><TT>pb->f()</TT></TD>
<TD>B::f() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pb->A::f()</TT></TD>
<TD>A::f() </TD>
<TD>B => A</TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pb->g()</TT></TD>
<TD>A::g() </TD>
<TD>B => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD>pb->h()</TD>
<TD>B::h()</TD>
<TD>none</TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa_in_b->f()</TT></TD>
<TD>B::f() </TD>
<TD>none </TD>
<TD>A => B</TD>
</TR>
<TR>
<TD><TT>pa_in_b->g()</TT></TD>
<TD>A::g() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa_in_b->h()</TT></TD>
<TD>B::h() </TD>
<TD>none </TD>
<TD>A => B</TD>
</TR>
<TR>
<TD><TT>pa_in_b->A::f()</TT></TD>
<TD>A::f() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD ROWSPAN="8">
<PRE><TT>struct C: public virtual A {
void g ();
void h ();
int ic;
};
C *pc;
A *pa_in_c = pc;</TT></PRE>
</TD>
<TD><TT>pc->f()</TT></TD>
<TD>A::f() </TD>
<TD>C => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pc->g()</TT></TD>
<TD>C::g() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pc->A::g()</TT></TD>
<TD>A::g() </TD>
<TD>C => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pc->h()</TT></TD>
<TD>C::h() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa_in_c->f()</TT></TD>
<TD>A::f() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa_in_c->g()</TT></TD>
<TD>C::g() </TD>
<TD>none</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD><TT>pa_in_c->h()</TT></TD>
<TD>C::h() </TD>
<TD>none</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD><TT>pa_in_c->A::g()</TT></TD>
<TD>A::g() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD ROWSPAN="21">
<PRE><TT>struct D: public B, public C {
int id;
void h();
};
D *pd;
A *pa_in_d = pd;
B *pb_in_d = pd;
C *pc_in_d = pd;
A *pa_in_b_in_d = pb_in_d;
A *pa_in_c_in_d = pc_in_d;</TT></PRE>
</TD>
<TD><TT>pd->f()</TT></TD>
<TD>B::f() </TD>
<TD>none [D => B] </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pd->g()</TT></TD>
<TD>C::g() </TD>
<TD>D => C </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pd->h()</TT></TD>
<TD>D::h() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pa_in_d->f()</TT></TD>
<TD>B::f() </TD>
<TD>none </TD>
<TD>A => B</TD>
</TR>
<TR>
<TD><TT>pa_in_d->g()</TT></TD>
<TD>C::g() </TD>
<TD>none</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD><TT>pa_in_d->h()</TT></TD>
<TD>D::h() </TD>
<TD>none</TD>
<TD>A => D</TD>
</TR>
<TR>
<TD><TT>pb_in_d->f()</TT></TD>
<TD>B::f() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pb_in_d->g()</TT></TD>
<TD>C::g() </TD>
<TD>B => A</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD><TT>pb_in_d->h()</TT></TD>
<TD>D::h() </TD>
<TD>none </TD>
<TD>B => D</TD>
</TR>
<TR>
<TD><TT>pc_in_d->f()</TT></TD>
<TD>B::f() </TD>
<TD>C => A</TD>
<TD>A => B</TD>
</TR>
<TR>
<TD><TT>pc_in_d->g()</TT></TD>
<TD>C::g() </TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pc_in_d->h()</TT></TD>
<TD>D::h() </TD>
<TD>none </TD>
<TD>C => D</TD>
</TR>
<TR>
<TD><TT>pa_in_b_in_d->f()</TT></TD>
<TD COLSPAN="3" ROWSPAN="6">same as for pa_in_d </TD>
</TR>
<TR>
<TD><TT>pa_in_b_in_d->g()</TT></TD>
</TR>
<TR>
<TD><TT>pa_in_b_in_d->h()</TT></TD>
</TR>
<TR>
<TD><TT>pa_in_c_in_d->f()</TT></TD>
</TR>
<TR>
<TD><TT>pa_in_c_in_d->g()</TT></TD>
</TR>
<TR>
<TD><TT>pa_in_c_in_d->h()</TT></TD>
</TR>
<TR>
<TD><TT>p...d->A::f()</TT></TD>
<TD>A::f() </TD>
<TD>... => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>p...d->A::g()</TT></TD>
<TD>A::g() </TD>
<TD>... => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>p...d->A::h()</TT></TD>
<TD>A::g() </TD>
<TD>... => A </TD>
<TD>none</TD>
</TR>
<TR>
<TD ROWSPAN="17">
<PRE><TT>struct X {
int ix;
virtual void x();
};</TT></PRE>
<PRE><TT>struct E : X, D {
int ie;
void f();
void h();
};</TT></PRE>
</TD>
</TR>
<TR>
<TD><TT>pe->f()</TT></TD>
<TD>E::f()</TD>
<TD>none</TD>
<TD>none</TD>
</TR>
<TR>
<TD><TT>pe->g()</TT></TD>
<TD>C::g()</TD>
<TD>E => C</TD>
<TD>none</TD>
</TR>
<TR>
<TD>pe->h()</TD>
<TD>E::h()</TD>
<TD>none </TD>
<TD>none</TD>
</TR>
<TR>
<TD>pe->x()</TD>
<TD>X::x()</TD>
<TD>none [E=>X]</TD>
<TD>none</TD>
</TR>
<TR>
<TD>
<PRE>pa_in_e->f()</PRE>
</TD>
<TD>E::f()</TD>
<TD>none</TD>
<TD>A => E</TD>
</TR>
<TR>
<TD>pa_in_e->g()</TD>
<TD>C::g()</TD>
<TD>none</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD>pa_in_e->h()</TD>
<TD>E::h()</TD>
<TD>none</TD>
<TD>A => E</TD>
</TR>
<TR>
<TD>pb_in_e->f()</TD>
<TD>E::f()</TD>
<TD>none</TD>
<TD>B => E</TD>
</TR>
<TR>
<TD>pb_in_e->g()</TD>
<TD>C::g()</TD>
<TD>B => A</TD>
<TD>A => C</TD>
</TR>
<TR>
<TD>pb_in_e->h()</TD>
<TD>E::h()</TD>
<TD>none </TD>
<TD>B => E</TD>
</TR>
<TR>
<TD>pc_in_e->f()</TD>
<TD>E::f()</TD>
<TD>C => A</TD>
<TD>A => E</TD>
</TR>
<TR>
<TD>pc_in_e->g()</TD>
<TD>C::g()</TD>
<TD>none</TD>
<TD>none</TD>
</TR>
<TR>
<TD>pc_in_e->h()</TD>
<TD>E::h()</TD>
<TD>none</TD>
<TD>C => E</TD>
</TR>
<TR>
<TD>pd_in_e->f()</TD>
<TD>E::f()</TD>
<TD>none [D=>B]</TD>
<TD>B => E</TD>
</TR>
<TR>
<TD>pd_in_e->g()</TD>
<TD>C::g()</TD>
<TD>D => C</TD>
<TD>none</TD>
</TR>
<TR>
<TD>pd_in_e->h()</TD>
<TD>E::h()</TD>
<TD>none</TD>
<TD>D => E</TD>
</TR>
</TABLE>
<HR>
<TABLE BORDER=2 CELLPADDING=4 >
<CAPTION>Table 1b: Example Data Layout </CAPTION>
<TR>
<TH>Declarations </TH>
<TH>Size </TH>
<TD><B>Offset</B></TD>
<TH>Member </TH>
</TR>
<TR>
<TD ROWSPAN="2">
<PRE><TT>struct A {
virtual void f ();
virtual void g ();
virtual void h ();
int ia;
};</TT></PRE>
</TD>
<TD ROWSPAN="2">16 </TD>
<TD>0</TD>
<TD><I><TT>A::vptr</TT></I></TD>
</TR>
<TR>
<TD>8 </TD>
<TD><TT>ia</TT></TD>
</TR>
<TR>
<TD ROWSPAN="4">
<PRE><TT>struct B: public virtual A {
void f ();
void h ();
int ib;
};</TT></PRE>
</TD>
<TD ROWSPAN="4">32 </TD>
<TD>0</TD>
<TD><I><TT>B::vptr</TT></I></TD>
</TR>
<TR>
<TD>8 </TD>
<TD><TT>ib</TT></TD>
</TR>
<TR>
<TD>16 </TD>
<TD><I><TT>A::vptr</TT></I></TD>
</TR>
<TR>
<TD>24 </TD>
<TD><TT>ia</TT></TD>
</TR>
<TR>
<TD ROWSPAN="4">
<PRE><TT>struct C: public virtual A {
void g ();
void h ();
int ic;
};</TT></PRE>
</TD>
<TD ROWSPAN="4">32 </TD>
<TD>0</TD>
<TD><I><TT>C::vptr</TT></I></TD>
</TR>
<TR>
<TD>8 </TD>
<TD><TT>ic</TT></TD>
</TR>
<TR>
<TD>16 </TD>
<TD><I><TT>A::vptr</TT></I></TD>
</TR>
<TR>
<TD>24 </TD>
<TD><TT>ia</TT></TD>
</TR>
<TR>
<TD ROWSPAN="7">
<PRE><TT>struct D: public B, public C {
void h ();
int id;
};</TT></PRE>
</TD>
<TD ROWSPAN="7">48 </TD>
<TD>0</TD>
<TD><I><TT>D/B::vptr</TT></I></TD>
</TR>
<TR>
<TD>8 </TD>
<TD><TT>ib</TT></TD>
</TR>
<TR>
<TD>16 </TD>
<TD><I><TT>C::vptr</TT></I></TD>
</TR>
<TR>
<TD>24 </TD>
<TD><TT>ic</TT></TD>
</TR>
<TR>
<TD>28 </TD>
<TD><TT>id</TT></TD>
</TR>
<TR>
<TD>32 </TD>
<TD><I><TT>A::vptr</TT></I></TD>
</TR>
<TR>
<TD>40 </TD>
<TD><TT>ia</TT></TD>
</TR>
<TR>
<TD ROWSPAN="10">
<PRE>s<TT>truct X {
int ix;
virtual void x();
};</TT></PRE>
<TT>struct E : X, D {</TT>
<BR><TT> void f ();</TT>
<BR><TT> void h ();</TT>
<BR><TT> int ie;</TT>
<BR><TT>};</TT></TD>
<TD ROWSPAN="10">72</TD>
<TD>0</TD>
<TD><I>X/E::vptr</I></TD>
</TR>
<TR>
<TD>8</TD>
<TD>ix</TD>
</TR>
<TR>
<TD>16</TD>
<TD><I>D/B::vptr</I></TD>
</TR>
<TR>
<TD>24</TD>
<TD>ib</TD>
</TR>
<TR>
<TD>32</TD>
<TD><I>C::vptr</I></TD>
</TR>
<TR>
<TD>40</TD>
<TD>ic</TD>
</TR>
<TR>
<TD>44</TD>
<TD>id</TD>
</TR>
<TR>
<TD>48</TD>
<TD>ie</TD>
</TR>
<TR>
<TD>56</TD>
<TD><I>A::vptr</I></TD>
</TR>
<TR>
<TD>64</TD>
<TD>ia</TD>
</TR>
</TABLE>
<HR>
<TABLE BORDER=2 CELLPADDING=4 >
<CAPTION>Table 1c: Example Vtable Layout </CAPTION>
<TR>
<TH>Declarations </TH>
<TH>Vtable (HP) <SUP>1,2,3</SUP></TH>
<TD><B>Vtable (Cygnus/IBM)</B></TD>
</TR>
<TR>
<TD>
<PRE><TT>struct A {
virtual void f ();
virtual void g ();
virtual void h ();
int ia;
};</TT></PRE>
</TD>
<TD>
<PRE><TT>A::offset_to_top (0)
A::rtti
-- A vtable address --
A::f() []
A::g() []
A::h() []</TT></PRE>
</TD>
<TD>
<PRE><TT>A::offset_to_top (0)
A::rtti
-- A vtable address --
A::f() []
A::g() []
A::h() []</TT></PRE>
</TD>
</TR>
<TR>
<TD>
<PRE><TT>struct B: public virtual A {
void f ();
void h ();
int ib;
};</TT></PRE>
</TD>
<TD>
<PRE><TT>B::offset_to_A (16)
B::offset_to_top (0)
B::rtti
-- B vtable address --
B::f() []
B::h() []
A::offset_to_top (-16)
A::rtti
-- A-in-B vtable address --
B::f() [[-72] B::offset_to_A : thunk]
A::g() []
B::h() [[-72] B::offset_to_A : thunk]</TT></PRE>
</TD>
<TD>
<PRE><TT>B::offset_to_A (16)
B::offset_to_top (0)
B::rtti
-- B vtable address --
B::f() []
B::h() []
A::offset_for_h (-16)
A::offset_for_g (0)
A::offset_for_f (-16)
A::offset_to_top (-16)
A::rtti
-- A-in-B vtable address --
B::f() [[-24]offset_for_f]
A::g() []
B::h() [[-40]offset_for_h]</TT></PRE>
</TD>
</TR>
<TR>
<TD>
<PRE><TT>struct C: public virtual A {
void g ();
void h ();
int ic;
};</TT></PRE>
</TD>
<TD>
<PRE><TT>C::offset_to_A (16)
C::offset_to_top (0)
C::rtti
-- C vtable address --
C::g() []
C::h() []
A::offset_to_top (-16)
A::rtti
-- A-in-C vtable address --
A::f() []
C::g() [[-72] C::offset_to_A : thunk]
C::h() [[-72] C::offset_to_A : thunk]</TT></PRE>
<PRE><TT>total size 15*8 = 120 bytes</TT></PRE>
</TD>
<TD>
<PRE><TT>C::offset_to_A (16)
C::offset_to_top (0)
C::rtti
C vtable address --
C::g() []
C::h() []
A::offset_for_h (-16)
A::offset_for_g (-16)
A::offset_for_f (0)
A::offset_to_top (-16)
A::rtti
A-in-C vtable address --
A::f() []
C::g() [[-32] offset_for_g]
C::h() [[-40] offset_for_h]</TT></PRE>
<PRE><TT>total size 18*8 = 144 bytes</TT></PRE>
</TD>