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<b>Data Structures and Algorithms 
with Object-Oriented Design Patterns in C++</b><br>
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<H3><A NAME="SECTION0018411000000000000000">Derivation and Access Control</A></H3>
<A NAME="seccppprotected">&#160;</A>
<P>
Members of a class can be <tt>private</tt>, <tt>public</tt> or <tt>protected</tt>.
As explained in Section&nbsp;<A HREF="page605.html#seccppaccess" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page605.html#seccppaccess"><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>,
private members are accessible only by member functions
and friends of the class in which the member is declared.
In particular,
this means that the member functions of a derived class cannot
access the private members of the base classes even though the derived
class has inherited those members!
On the other hand,
if we make the members of the base classes public,
then all classes can access those members directly,
not just derived classes.
<P>
C++ provides a third category of access control--<tt>protected</tt>.
<tt>Protected</tt> members can be used by member functions
and friends of the class in which the member is declared
as well as by member functions and friends of all the classes derived
from the class in which the member is declared.
<P>
A derived class <em>inherits</em> all the members of its base class(es).
The accessibility of the members inherited by a derived class
depends on whether the derivation is <tt>private</tt> or <tt>public</tt>:
<PRE>class B { ... };
class D1 : public B { ... };
class D2 : private B { ... };</PRE>
Here <tt>B</tt> is a public base class of <tt>D1</tt>.
This means that the public members of <tt>B</tt>
are also public in <tt>D1</tt>
and that the protected members of <tt>B</tt>
are also protected in <tt>D1</tt>.
On the other hand, <tt>B</tt> is a private base class of <tt>D2</tt>.
This means that the public and protected members of <tt>B</tt>
are private in <tt>D2</tt>.
<P>
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