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《Big C++ 》Third Edition电子书和代码全集-Part1

<pre>class TeachingAssistant : public Student, public Employee
{
   . . .
}</pre>
						</font>
					</td>
				</tr>
				<tr>
					<td valign="top"><font size="+1" color="#009999">Purpose:</font></td>
					<td><font size="+1">Define a class that inherits features from two or
					more base classes.</font></td>
				</tr>
			</table>
			</font>
		</td>
	</tr>
</table>

</font>

<hr><h2><font color="#009999">21.7 Muliple Inheritance (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Instances of derived classes maintain data fields of all base
		classes:
		<script><!--
			image( "fig07.png" )
		//--></script>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7 Muliple Inheritance (cont.)</font></h2>
<font size="+1">

<ul>
	<li>The idea of polymorphic assignment applies for each of the base classes:
		<blockquote><tt>
			TeachingAssistant* fred = new TeachingAssistant();
			<br>Employee* new_hire = fred;<font color="#0000cc">
				// Legal, because a TeachingAssistant is-a Employee</font>
			<br>Student* advisee = fred;<font color="#0000cc">
				// Legal, because a TeachingAssistant is-a Student</font>
		</tt></blockquote>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7 Muliple Inheritance (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Dynamic cast and <tt>typeid</tt> work just as we'd like:
		<blockquote>
<pre>Student* mary = . . .;
TeachingAssistant* lab_instructor
   = dynamic_cast&lt;TeachingAssistant*&gt;(mary);
if (lab_instructor != NULL)
   cout &lt;&lt; "Yes, mary is a TeachingAssistant. \n";
else
   cout &lt;&lt; "No, mary is not a TeachingAssistant. \n";</pre>
		</blockquote>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7.1 Name Ambiguity</font></h2>
<font size="+1">

<ul>
	<li>Names of inherited operations may clash</li>
	<li>E.g., suppose both <tt>Employee</tt> and <tt>Student</tt> provide a
		<tt>get_id</tt> method, that refer to different IDs:
		<blockquote><tt>
			TeachingAssistant* fred = new TeachingAssistant();
			<br><font color="#cc0000">cout &lt;&lt; "Your number is " &lt;&lt;
				fred-&gt;get_id() &lt;&lt; "\n";</font>
			<br><font color="#0000cc">// Error, ambiguous member function name</font>
		</tt></blockquote>
	</li>
	<li>2 resolutions:</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7.1 Name Ambiguity (cont.)</font></h2>
<font size="+1">

<ol>
	<li>Use the fully qualified function name:
		<blockquote><tt>
			cout &lt;&lt; "Your teaching assistant is "<br>
				&nbsp;&nbsp;&nbsp;&lt;&lt;
				<nobr>fred-&gt;Employee::get_id()</nobr> &lt;&lt; "\n";
		</tt></blockquote>
	</li>
</ol>

</font>

<hr><h2><font color="#009999">21.7.1 Name Ambiguity (cont.)</font></h2>
<font size="+1">

<ol start='2'>
	<li>Redefine the ambiguous name in the new class:
		<blockquote>
<pre>class TeachingAssistant : public Student, public Employee
{
public:
   string get_id() const;
   string student_id() const;
};</pre>
		</blockquote>
	</li>
</ol>

</font>

<hr><h2><font color="#009999">21.7.1 Name Ambiguity (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Hide the use of the qualified name within the body</li>
</ul>

<blockquote>
<pre>// get_id will return Employee identification number
string TeachingAssistant::get_id()
{
   return Employee::get_id();
}

string TeachingAssistant::student_id()
	// Make student value available by a different name
{
   return Student::get_id();
}</pre>
</blockquote>
</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes</font></h2>
<font size="+1">

<ul>
	<li>Base class is specified only once</li>
	<li>A class may be inherited (indirectly) more than once</li>
	<li>E.g., <tt>Person</tt> as a base class for <tt>Employee</tt> and
		<tt>Student</tt>:
		<blockquote>
<pre>class Person
{
public:
   Person(string n);
   string get_name() const;
private:
   string name;
};</pre>
		</blockquote>
	</li>
</ul>
</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes</font></h2>
<font size="+1">

<blockquote>
<pre>class Employee : public Person
{
   . . .
};

class Student : public Person
{
   . . .
};</pre>
</blockquote>

</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes (cont.)</font></h2>
<font size="+1">

<ul>
	<li><tt>TeachingAssistant</tt> now has 2 <tt>name</tt> fields:
		<script><!--
			image( "un03.png" )	// page 781, 3
		//--></script>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes (cont.)</font></h2>
<font size="+1">

<ul>
	<li>If this is not desirable, specify inheritance as <tt>virtual</tt>:
		<blockquote>
<pre>class Student : <font color="#009999">virtual</font> public Person
{
   . . .
};

class Employee : <font color="#009999">virtual</font> public Person
{
   . . .
};

class TeachingAssistant : public Student, public Employee
{
   . . .
};</pre>
		</blockquote>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes (cont.)</font></h2>
<font size="+1">

<ul>
	<li>The intermediate classes need to use virtual inheritance</li>
	<li>Virtual inheritance alters placement of base classes</li>
		<script><!--
			image( "fig08.png" )	// page 782, 3
		//--></script>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Duplicated data areas from the common base class are eliminated:
		<script><!--
			image( "fig09.png" )	// page 782, 3
		//--></script>
</ul>

</font>

<hr><h2><font color="#009999">21.7.2 Replicated Base Classes (cont.)</font></h2>
<font size="+1">

<ul>
	<li><tt>TeachingAssistant*</tt> can always be converted to an
		<tt>Employee*</tt>, then to a <tt>Person*</tt></li>
	<li>Without virtual inheritance, a <tt>TeachingAssistant*</tt> can <b>not</b>
		be converted directly to a <tt>Person*</tt>:
		<blockquote><tt>
			TeachingAssistant* fred = new TeachingAssistant();
			<br>Student* s = fred;<font color="#0000cc">
				// Legal, because TeachingAssistant <nobr>is-a</nobr> Student</font>
			<br>Person* p1 = s;
			<br>Employee* e = fred;
			<br>Person* p2 = e;
			<br><font color="#009999">Person* p3 = fred;</font><font color="#0000cc">
				// Legal if virtual inheritance used, otherwise error</font>
		</tt></blockquote>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.8 Software Frameworks</font></h2>
<font size="+1">

<ul>
	<li>Modern programming emphasized software reuse</li>
	<li>A reusable component must be general purpose</li>
	<li>Details of an individual application are required</li>
	<li>Polymorphism provides a handy solution:
		<ul>
			<li>A general base class is developed and distributed across many
				projects</li>
			<li>Use Inheritance to create specialized versions of the general base
				class</li>
		</ul>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.8 Software Frameworks (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Consider the creation of GUI:</li>
		<ul>
			<li>Many different components</li>
			<li>Each component may have complex behavior</li>
		</ul>
	</li>
	<li>Rewriting this behavior for each new application would be difficult</li>
	<li>Programmers employ a <i>software framework</i>, or <i>toolkit</i>:</li>
		<ul>
			<li>Collection of reusable classes and functions</li>
			<li>A starting point used in the creation of new applications</li>
		</ul>
	</li>
</ul>

</font>

<hr><h2><font color="#009999">21.8 Software Frameworks (cont.)</font></h2>
<font size="+1">

<p>Consider putting a window up on a screen.  The concept of window is
generic.</p>

<ul>
	<li>Put code to handle generic tasks, such as moving the window, in the base
		class</li>
	<li>For application-aware tasks, base class provides simple default behavior
		or pure virtual functions:</li>
</ul>

</font>

<hr><h2><font color="#009999">21.8 Software Frameworks (cont.)</font></h2>
<font size="+1">

<blockquote>
<pre>class wxWindow
{
public:
   . . .
   <font color="#0000cc">// Pure virtual member function</font>
   virtual void OnPaint(wxPaintEvent&amp; event) = 0;
   virtual void OnMouseEvent(wxMouseEvent&amp; event);
   . . .
};

void wxWindow::OnMouseEvent(wxMouseEvent&amp; event)
{
   <font color="#0000cc">// Default behavior, do nothing</font>
}</pre>
</blockquote>

</font>

<hr><h2><font color="#009999">21.8 Software Frameworks (cont.)</font></h2>
<font size="+1">

<ul>
	<li>Each new application inherits from <tt>wxWindow</tt> to create a special
	purpose class</li>
	<li>Application-specific details can be filled in:</li>
</ul>
</font>

<hr><h2><font color="#009999">21.8 Software Frameworks (cont.)</font></h2>
<font size="+1">

<blockquote>
<pre>class ClockWindow : public wxWindow
   <font color="#0000cc">// ClockWindow will be developed in Chapter 27</font>
{
public:
   . . .
   virtual void OnPaint(wxPaintEvent&amp; event);
   virtual void OnMouseEvent(wxMouseEvent&amp; event);
   . . .
};

void ClockWindow::OnMouseEvent(wxMouseEvent&amp; event)
{
   . . . <font color="#0000cc">// Implement mouse events for this specific application</font>
}</pre>
</blockquote>

</font>

<hr><h2><font color="#009999">Chapter Summary</font></h2>
<font size="+1">
<hr color="#00ffff" size="6">

<ol>
	<li>Inheritance can be used to organize classes into hierarchies.</li>
	<li>Polymorphic variables are very powerful.  Declared as one type, its
	static type, but can maintain a value of a different type, its dynamic type.
	Must be pointers or references.</li>
	<li>When a virtual method is called using a polymorphic variable, its
	dynamic type is used.
	<li>A virtual method that is declared but not defined is called a pure
	virtual method.  A class with a pure virtual method can not be instantiated.
	It is called an abstract class.</li>
</ol>

</font>

<hr><h2><font color="#009999">Chapter Summary (cont.)</font></h2>
<font size="+1">
<hr color="#00ffff" size="6">

<ol start='5'>
	<li>The dynamic type of a polymorphic variable can be tested using a dynamic
	cast, or the <tt>typeid</tt> operator.</li>
	<li>A class that inherits from two or more base classes is said to use
	multiple inheritance.</li>
	<li>A software framework (or toolkit) is a collection of classes and/or
	functions that capture the common features of a task.  Methods are overridden
	to specialize behavior.</li>
</ol>

</font>

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