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   <TITLE> What's Wrong With This Code? Volume #4</TITLE>

   <META NAME="Author" CONTENT="Harold Howe">

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<H2>

What's Wrong With This Code? Volume #4

</H2>



<H4>

Base class destructors

</H4>

<P>

The code example below contains three classes: <TT>Foo</TT>, <TT>Base</TT>, and

<TT>Derived</TT>. The <TT>Derived</TT> class contains two data members that are <TT>Foo</TT>

objects. One is a pointer, and one is not. <TT>Derived</TT> publically inherits from

<TT>Base</TT>.

</P>

<pre>

<font color="green">#include &lt;iostream&gt;</font>



<b>using</b> <b>namespace</b> std<b>;</b>



<b>class</b> Foo

<b>{</b>

<b>public</b><b>:</b>

    Foo<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;Foo constructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

    <b>}</b>

    <b>~</b>Foo<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;Foo destructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

    <b>}</b>

<b>}</b><b>;</b>



<b>class</b> Base

<b>{</b>

<b>public</b><b>:</b>

    Base<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;base constructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

    <b>}</b>



    <b>~</b>Base<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;base destructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

    <b>}</b>

<b>}</b><b>;</b>



<b>class</b> Derived <b>:</b> <b>public</b> Base

<b>{</b>

    Foo  foo1<b>;</b>

    Foo <b>*</b>foo2<b>;</b>

<b>public</b><b>:</b>

    Derived<b>(</b><b>)</b>

      <b>:</b>Base<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;derived constructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

        foo2 <b>=</b> <b>new</b> Foo<b>;</b>

    <b>}</b>



    <b>~</b>Derived<b>(</b><b>)</b>

    <b>{</b>

        cout <b>&lt;&lt;</b> <font color="blue">&quot;derived destructor&quot;</font> <b>&lt;&lt;</b> endl<b>;</b>

        <b>delete</b> foo2<b>;</b>

    <b>}</b>

<b>}</b><b>;</b>



<font color="navy">//---------------------------------------------------------------------------</font>

<b>int</b> main<b>(</b><b>)</b>

<b>{</b>

    Base <b>*</b>b <b>=</b> <b>new</b> Derived<b>;</b>

    <b>delete</b> b<b>;</b>

    <b>return</b> <font color="blue">0</font><b>;</b>

<b>}</b>

</pre>



<P>

The output from this program is:

</P>

<pre>

base constructor

Foo constructor

derived constructor

Foo constructor

base destructor

</pre>

<P>

The program seems to run OK. It does not crash and burn, and it does not hang the operating

system. On Windows, that's always a major accomplishment. However, the output from the program indicates that

something is wrong. Can you find the problem?

</P>



<H4>

<A TARGET=_top href="wwwtc4answer.htm">Answer</A>

</H4>

<BR>

<TABLE  BORDER=1 CELLPADDING=10 CELLSPACING=0 WIDTH="100%">

<TR> <TD colspan = 2><B>Code for this edition</B> </TD> </TR>

<TR> <TD><TT><a HREF="download/wwwtc4.zip">wwwtc4.zip</a></TT></TD><TD>BCB4 project that contains problem code.</TD> </TR>

<TR> <TD><TT><a HREF="download/wwwtc4x.zip">wwwtc4x.zip</a></TT></TD><TD>Same project, includes an EXE (55 kb).</TD> </TR>

</TABLE>





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