0481-0483.html
来自「linux-unix130.linux.and.unix.ebooks130 l」· HTML 代码 · 共 338 行
HTML
338 行
<HTML>
<HEAD>
<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:C and C++ Programming</TITLE>
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
<SCRIPT>
<!--
function displayWindow(url, width, height) {
var Win = window.open(url,"displayWindow",'width=' + width +
',height=' + height + ',resizable=1,scrollbars=yes');
}
//-->
</SCRIPT>
</HEAD>
-->
<!-- ISBN=0672311739 //-->
<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS PUBLISHING //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=23 //-->
<!-- PAGES=0455-0486 //-->
<!-- UNASSIGNED1 //-->
<!-- UNASSIGNED2 //-->
<P><CENTER>
<a href="0478-0480.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0484-0486.html">Next</A>
</CENTER></P>
<A NAME="PAGENUM-481"><P>Page 481</P></A>
<!-- CODE //-->
<PRE>
newlicensee.zipcode = "98765";
newlicensee.height = 70;
newlicensee.weight = 165;
license.month = 1;
newlicensee.day = 23;
newlicensee.year = 97;
newlicensee.license_letter = A;
newlicensee.license_number = 567890;
newlicensee.display; // and display this instance of the structure
}
</PRE>
<!-- END CODE //-->
<P>Note that there are three references to the same
display function in Listing 23.13. First, the
display function is prototyped as an element within the structure definition. Second, the
function is defined. Because the function definition is valid for all instances of the datatype
license, the structure's data elements are referenced by the
display function without naming any instance of the structure. Finally, when a specific instance of
license is created, its associated display function is invoked by prefixing the function name with that of the
structure instance.
</P>
<P>Listing 23.14 shows the output of this program.
</P>
<P>Listing 23.14. Output of the function defined within a structure.
</P>
<!-- CODE //-->
<PRE>
Name: Joe Smith
Address: 123 Elm Street
Smalltown, AnyState 98765
Height: 70 inches
Weight: 160 lbs
Date: 1/23/1997
License: A567890
</PRE>
<!-- END CODE //-->
<P>Note that the operator << is the bitwise shift left operator except when it is used with
cout. With cout, << is used to move data to the screen. This is an example of operator
overloading because the operator can have a different meaning depending on the context of its use. The
>> operator is used for bitwise shift right except when used with
cin; with cin, it is used to move data from the keyboard to the specified
variable.
</P>
<H4><A NAME="ch23_ 25">
Classes in C++
</A></H4>
<P>Overloading and associating functions with data structures lay the groundwork for object-
oriented code in C++. Full object orientation is available through the use of the C++ class
feature.
</P>
<P>A C++ class extends the idea of data structures with associated functions by binding (or
encapsulating) data descriptions and manipulation algorithms into new abstract datatypes. When
a class is defined, the class type and methods are described in the public interface. The class
can also have hidden private functions and data members as well.
</P>
<A NAME="PAGENUM-482"><P>Page 482</P></A>
<P>Class declaration defines a datatype and format, but does not allocate memory or in any
other way create an object of the class's type. The wider program must declare an instance, or
object, of this type in order to store values in the data elements or to invoke the public class
functions. A class is often placed into libraries for use by many different programs, each of which
then declares objects that instantiate that class for use during program
execution.
</P>
<H4><A NAME="ch23_ 26">
Declaring a Class in C++
</A></H4>
<P>Listing 23.15 contains an example of a typical class declaration in C++.
</P>
<P>Listing 23.15. Declaring a class in C++.
</P>
<!-- CODE //-->
<PRE>
#include <iostream.h>
// declare the Circle class
class Circle {
private:
double rad; // private data member
public:
Circle (double); // constructor function
~Circle (); // deconstructor function
double area (void); // member function - compute area
};
// constructor function for objects of this class
Circle::Circle(double radius)
{
rad = radius;
}
// deconstructor function for objects of this class
Circle::~Circle()
{
// does nothing
}
// member function to compute the Circle's area
double Circle::area()
{
return rad * rad * 3.141592654;
}
// application program that uses a Circle object
main()
{
Circle mycircle (2); // declare a circle of radius = 2
cout << mycircle.area(); // compute & display its area
}
</PRE>
<!-- END CODE //-->
<P>The example in Listing 23.15 begins by declaring the
Circle class. This class has one private member, a floating-point element. The
Circle class also has several public members,
consisting of three functions—Circle, ~Circle, and
area.
</P>
<P>The constructor function of a class is a function called by a program in order to construct
or create an object that is an instance of the class. In the case of the
Circle class, the constructor
</P>
<A NAME="PAGENUM-483"><P>Page 483</P></A>
<P>function (Circle(double)) requires a single parameter, namely the radius of the desired
circle. If a constructor function is explicitly defined, it has the same name as the class and does
not specify a return value, even of type void.
</P>
<P>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
NOTE
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
When a C++ program is compiled, the compiler generates calls to the runtime
system, which allocates sufficient memory each time an object of class
Circle comes into scope. For example, an object that is defined within a function is created (and goes into
scope) whenever the function is called. However, the object's data elements are not
initialized unless a constructor function has been defined for the class.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<P>The deconstructor function of a class is a function called by a program in order to deconstruct
an object of the class type. A deconstructor takes no parameters and returns nothing. In this
example, the Circle class's deconstructor function is
~Circle.
</P>
<P>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
NOTE
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
Under normal circumstances, the memory associated with an object of a given class
is released for reuse whenever the object goes out of scope. In such a case, the
programmer can omit defining the deconstructor function. However, in advanced applications
or where class assignments cause potential pointer conflicts, explicit deallocation of
free-store memory might be necessary.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<P>In addition to the constructor and deconstructor functions, the
Circle class contains a public function called
area. Programs can call this function to compute the area of
Circle objects.
</P>
<P>The main program (the main function) in Listing 23.15 shows how an object can be
declared. mycircle is declared to be of type
Circle and is given a radius of 2.
</P>
<P>The final statement in this program calls the function to compute the area of
mycircle and passes it to the output function for display. Note that the area computation function is
identified by a composite name, just as with other functions that are members of C++ data
structures outside of class definitions. This usage underscores the fact that the object
mycircle, of type Circle, is being asked to execute a function that is a member of itself, and with reference
to itself. The programmer could define a Rectangle class that also contains an
area function, thereby overloading the area function name with the appropriate algorithm for computing the areas
of different kinds of geometric entities.
</P>
<H4><A NAME="ch23_ 27">
Inheritance and Polymorphism
</A></H4>
<P>A final characteristic of object-oriented languages, and of C++, is support for class
inheritance and for polymorphism.
</P>
<P><CENTER>
<a href="0478-0480.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0484-0486.html">Next</A>
</CENTER></P>
</td>
</tr>
</table>
<!-- begin footer information -->
</body></html>⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?