📄 atl under the hood - part 2.mht
字号:
address of=20
VTable.=20
<H3>Program 23</H3><PRE><SPAN class=3Dcpp-preprocessor>#include =
<iostream></SPAN>
using <SPAN class=3Dcpp-keyword>namespace</SPAN> std;
<SPAN class=3Dcpp-keyword>class</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Base() {
cout << <SPAN class=3Dcpp-string>"In Base"</SPAN> << =
endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Base::f1"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>class</SPAN> Drive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Drive() {
cout << <SPAN class=3Dcpp-string>"In Drive"</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Drive::f2"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>int</SPAN> main() {
Drive d;
<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>The output of this program is <PRE>In Base
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C08C
Value at Vtable =3D 004010F0
In Drive
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C07C
Value at Vtable =3D <SPAN class=3Dcpp-literal>00401217</SPAN>
</PRE>This program shows the different vtable address in Base class and=20
Drive class. To get more better understanding lets make =
inheritance deeper=20
and add one more class <CODE>MostDrive</CODE> inherited from Drive =
and=20
make an object of it.=20
<H3>Program 24</H3><PRE><SPAN class=3Dcpp-preprocessor>#include =
<iostream></SPAN>
using <SPAN class=3Dcpp-keyword>namespace</SPAN> std;
<SPAN class=3Dcpp-keyword>class</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Base() {
cout << <SPAN class=3Dcpp-string>"In Base"</SPAN> << =
endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Base::f1"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>class</SPAN> Drive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Drive() {
cout << <SPAN class=3Dcpp-string>"In Drive"</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Drive::f2"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>class</SPAN> MostDrive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Drive {
<SPAN class=3Dcpp-keyword>public</SPAN>:
MostDrive() {
cout << <SPAN class=3Dcpp-string>"In MostDrive"</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"MostDrive::f2"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>int</SPAN> main() {
MostDrive d;
<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>The output of this program is <PRE>In Base
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C0A0
Value at Vtable =3D 004010F5
In Drive
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C090
Value at Vtable =3D <SPAN class=3Dcpp-literal>00401221</SPAN>
In MostDrive
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C080
Value at Vtable =3D <SPAN class=3Dcpp-literal>00401186</SPAN>
</PRE>This program shows that virtual pointer in initialized in the=20
constructor of each class. Therefore the address of Vtable is =
different in=20
each class constructor and main use the vtable of most drive class =
in=20
inheritance chain whose object is created.=20
<P>Now see what each class constructor place in vtable. To do this =
take=20
pointer to function and store value of first entry of vtable in =
that=20
function pointer and try to execute it.=20
<H3>Program 25</H3><PRE><SPAN class=3Dcpp-preprocessor>#include =
<iostream></SPAN>
using <SPAN class=3Dcpp-keyword>namespace</SPAN> std;
<SPAN class=3Dcpp-keyword>typedef</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN>(*Fun)();
<SPAN class=3Dcpp-keyword>class</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Base() {
cout << <SPAN class=3Dcpp-string>"In Base"</SPAN> << =
endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
Fun pFun =3D (Fun)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN>;
pFun();
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Base::f1"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>class</SPAN> Drive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
Drive() {
cout << <SPAN class=3Dcpp-string>"In Drive"</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
=20
Fun pFun =3D (Fun)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN>;
pFun();
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"Drive::f1"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>class</SPAN> MostDrive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Drive {
<SPAN class=3Dcpp-keyword>public</SPAN>:
MostDrive() {
cout << <SPAN class=3Dcpp-string>"In MostDrive"</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Virtual Pointer =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)<SPAN =
class=3Dcpp-keyword>this</SPAN> << endl;
cout << <SPAN class=3Dcpp-string>"Address of Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
cout << <SPAN class=3Dcpp-string>"Value at Vtable =3D =
"</SPAN> << (<SPAN class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN> =
<< endl;
=20
Fun pFun =3D (Fun)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)*(<SPAN =
class=3Dcpp-keyword>int</SPAN>*)<SPAN class=3Dcpp-keyword>this</SPAN>;
pFun();
cout << endl;
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> f1() { cout << <SPAN =
class=3Dcpp-string>"MostDrive::f1"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>int</SPAN> main() {
MostDrive d;
<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>The output of this program is <PRE>In Base
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C098
Value at Vtable =3D 004010F5
Base::f1
In Drive
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C088
Value at Vtable =3D <SPAN class=3Dcpp-literal>00401221</SPAN>
Drive::f1
In MostDrive
Virtual Pointer =3D 0012FF7C
Address of Vtable =3D 0046C078
Value at Vtable =3D <SPAN class=3Dcpp-literal>00401186</SPAN>
MostDrive::f1
</PRE>This program shows that constructor of each class fills the vtable =
entries with their own virtual function. So <CODE>Base</CODE> =
class fills=20
the vtable with the address of the <CODE>Base</CODE>'s virtual =
functions=20
and when <CODE>Drive</CODE> class's constructor executes it will =
create=20
another vtable and store the virtual functions address.=20
<P>Now see the situation when there is more than one virtual =
function in=20
the base class and drive class override not all of these.=20
<H3>Program 26</H3><PRE><SPAN class=3Dcpp-preprocessor>#include =
<iostream></SPAN>
using <SPAN class=3Dcpp-keyword>namespace</SPAN> std;
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