📄 atl under the hood - part 2.mht
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<DIV id=3Dcontentdiv><!-- Article Starts -->
<H2>Introduction</H2>
<P>In this series of tutorials I am going to discuss some of the =
inner=20
workings of ATL and the techniques that ATL uses. This is the =
second=20
article in the series.</P>
<P>Let's explore some more interesting stuff behind the virtual =
function.=20
To make things consistent I am going to take the same number =
sequence and=20
start my discussion with Program 20.=20
<P>Let's take a look at the following Program=20
<H3>Program 20</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>:
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> fun() {
cout << <SPAN class=3Dcpp-string>"Base::fun"</SPAN> =
<< endl;
}
<SPAN class=3Dcpp-keyword>void</SPAN> show() {
fun();
}
};
<SPAN class=3Dcpp-keyword>class</SPAN> Drive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> fun() {
cout << <SPAN class=3Dcpp-string>"Drive::fun"</SPAN> =
<< endl;
}
};
<SPAN class=3Dcpp-keyword>int</SPAN> main() {
Drive d;
d.show();
<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>The output of the program is <PRE>Drive::fun</PRE>This program =
clearly shows how the base class's=20
function calls the drive class function if that function is =
virtual. This=20
technique is used in different frameworks like MFC and design =
pattern like=20
Template Design Pattern. Now change program little bit to see its=20
behavior. Now I m going to call virtual function from constructor =
of Base=20
class rather than member function.=20
<H3>Program 21</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() {
fun();
}
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> fun() {
cout << <SPAN class=3Dcpp-string>"Base::fun"</SPAN> =
<< endl;
}
};
<SPAN class=3Dcpp-keyword>class</SPAN> Drive : <SPAN =
class=3Dcpp-keyword>public</SPAN> Base {
<SPAN class=3Dcpp-keyword>public</SPAN>:
<SPAN class=3Dcpp-keyword>virtual</SPAN> <SPAN =
class=3Dcpp-keyword>void</SPAN> fun() {
cout << <SPAN class=3Dcpp-string>"Drive::fun"</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>Base::fun
</PRE>This program shows that we can't call a virtual function of a=20
derived class from constructor of a base class. Ok to see =
what is=20
going on under the hood let's print the value of this pointer in =
both=20
constructors. To make things simple remove other functions from =
the=20
classes.=20
<H3>Program 22</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>"This Pointer =3D =
"</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> f() { cout << <SPAN =
class=3Dcpp-string>"Base::f"</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>"This Pointer =3D =
"</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> f() { cout << <SPAN =
class=3Dcpp-string>"Drive::f"</SPAN> << endl; }
};
<SPAN class=3Dcpp-keyword>int</SPAN> main() {
Drive d;
cout << <SPAN class=3Dcpp-string>"In Main"</SPAN> << =
endl;
cout << (<SPAN class=3Dcpp-keyword>int</SPAN>*)&d << =
endl;
<SPAN class=3Dcpp-keyword>return</SPAN> <SPAN =
class=3Dcpp-literal>0</SPAN>;
}
</PRE>The output of the program is <PRE>In Base
This Pointer =3D 0012FF7C
In Drive
This Pointer =3D 0012FF7C
In Main
0012FF7C
</PRE>This shows that there is only one object in the memory location. =
Now=20
let's print the value at this pointer, i.e. value of vptr and =
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