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<b>Data Structures and Algorithms 
with Object-Oriented Design Patterns in C++</b><br>
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<H2><A NAME="SECTION0018430000000000000000">Multiple Inheritance</A></H2>
<P>
In C++ a class can be derived from one or more base classes.
However, all the base classes must be distinct.
I.e., the following declaration is not allowed:
<PRE>class D : public B, public B { ... }; // Wrong.</PRE>
Nevertheless, it is possible for a class to be <em>indirectly derived</em>
from the same base class more than once.
E.g., consider the following class definitions:
<PRE>class A { ... };
class B1 : public A { ... };
class B2 : public A { ... };
class D : public B1, public B2 { ... };</PRE>
The derived class <tt>D</tt> inherits
two instances of the base class <tt>A</tt>--one indirectly via <tt>B1</tt>,
the other indirectly via <tt>B2</tt>.
In particular,
this means that <tt>D</tt> contains two copies of each member variable
of class <tt>A</tt>.
It also means that when a member function of <tt>A</tt> is called,
it is necessary to specify which of the two instances of <tt>A</tt>
is to be used--the one from which <tt>B1</tt> is derived
or the one from which to <tt>B2</tt> is derived.
This scenario is shown in Figure&nbsp;<A HREF="page613.html#figcpp2" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page613.html#figcpp2"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>&nbsp;(a).
<P>
<P><A NAME="57912">&#160;</A><A NAME="figcpp2">&#160;</A> <IMG WIDTH=575 HEIGHT=185 ALIGN=BOTTOM ALT="figure57706" SRC="img2609.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2609.gif"  ><BR>
<STRONG>Figure:</STRONG> Multiple Derivation and Virtual Base Classes<BR>
<P>
<P>
Sometimes it makes more sense for the derived class <tt>D</tt>
to contain only one instance of the base class <tt>A</tt>.
In C++ this is accomplished
using <em>virtual base classes</em><A NAME=57918>&#160;</A>:
<PRE>class A { ... };
class B1 : virtual public A { ... };
class B2 : virtual public A { ... };
class D : public B1, public B2 { ... };</PRE>
In this case, the derived class <tt>D</tt> contains only one instance
of the base class <tt>A</tt>.
Therefore, <tt>D</tt> contains only one copy of the member variables
declared in <tt>A</tt>
and there is not ambiguity when invoking the member functions of <tt>A</tt>.
This corresponds to the situation shown in Figure&nbsp;<A HREF="page613.html#figcpp2" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page613.html#figcpp2"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>&nbsp;(b).
<P>
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