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Review</TITLE>
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<B> Next:</B> <A NAME=tex2html2568 HREF="node53.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node53.html">5.2 CC++
Introduction</A>
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<H1><A NAME=SECTION03210000000000000000>5.1 C++
Review</A></H1>
<P>
<A NAME=seccpp> </A>
<P>
We first review some of the basic C++
constructs used in the rest of
this chapter, so as to make subsequent material understandable to
readers familiar with C but not C++
. Readers familiar with C++
can
skip this section.
<P>
With a few exceptions, C++
is a pure extension of ANSI C. Most
valid ANSI C programs are also valid C++
programs. C++
extends C
by adding strong typing and language support for data abstraction and
object-oriented programming.
<P>
<H2><A NAME=SECTION03211000000000000000>5.1.1 Strong Typing and Memory Management</A></H2>
<P>
ANSI standard C introduced function prototypes to the C language. A
function prototype defines the type of each function argument and the
function's return value (the function's <em> signature
</em>). For
example:
<P>
<PRE>/* A forward declaration to a_function */
int a_function(float b, double c);
/* The definition of a_function */
int a_function(float b, double c) {
/* Function body */
}
</PRE>
<P>
C++
requires that function prototypes be provided for all functions
before they are used and enforces consistent function use between program
<A NAME=6682> </A>
files. Thus, it is possible to distinguish between functions that have
<A NAME=6683> </A>
the same name but different signatures. C++
uses this capability to
allow function names to be <em> overloaded</em>. That is, more than one
function can be defined with the same name; the compiler compares
function call arguments with function signatures to determine which
version to use.
<P>
In C programs, the library routines <tt> malloc</tt> and <tt> free</tt> are
used for dynamic memory allocation. C++
defines two additional
operators, <tt> new</tt> and <tt> delete</tt>, as illustrated in the following
code fragments.
<P>
<PRE>struct S {
/* Structure body */
};
S *sPtr = new S; /* Allocate instance of S */
delete sPtr; /* Delete instance of S */
int *iPtr = new int[25]; /* Allocate array of integers */
delete [] iPtr; /* Delete array of integers */
</PRE>
<P>
Notice that <tt> new</tt> is given a description of the type of the data
object to be allocated; it returns a pointer to dynamically allocated
data of that type. The <tt> delete</tt> operator is used to release
dynamically allocated storage. The programmer must indicate when an
array of objects is being deleted.
<P>
<H2><A NAME=SECTION03212000000000000000>5.1.2 Classes</A></H2>
<P>
<A NAME=6706> </A>
The most significant feature that C++
adds to C is the concept of
<A NAME=6707> </A>
classes. A class can be thought of as a generalization of a C
<A NAME=6708> </A>
structure. In C, a structure groups together data elements of various
<A NAME=6709> </A>
types under a single name; in C++
, structures can also contain <em>
member functions</em>. Like data elements of a C structure, member
functions of a class can be accessed only through a reference to an
object of the appropriate type. In C++
, a class defines a scope in
which names referring to functions and data can be defined. Classes
can be introduced using the C keywords <tt> struct</tt> and <tt> union</tt>
or the
C++
keyword <tt> class</tt>.
<P>
<P><A NAME=progccex2> </A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img831.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img831.gif"><P>
<P>
Program <A HREF="node52.html#progccex2" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node52.html#progccex2">5.1</A> illustrates various features
of the C++
class mechanism. This program defines a class named <tt>
Datum</tt> containing a data member <tt> x</tt>, a member function <tt>
get_x</tt>, and two constructor functions. (Notice the
C++
single-line comments; anything after a double slash <tt> //</tt>
is a comment.) These terms are defined in the following discussion.
<P>
The syntax <tt> Datum::get_x()</tt> is used to name a member function
<tt> get_x</tt> of <tt> Datum</tt>. This name, called a <em> quantified
name</em>, specifies that we are referring to a function defined in the
scope of <tt> Datum</tt>. If we do not quantify the name, we are defining
the <em> global
</em> function <tt> get_x()</tt>, which is a different
function. Notice that within the definition of <tt> Datum::get_x()</tt>
we can refer to the data member <tt> x</tt> directly, because <tt> x</tt> and
<tt> get_x</tt> are defined in the same scope. We also could incorporate
the definition for function <tt> get_x</tt> directly in the class
definition, as follows.
<P>
<PRE>public:
int get_x() { return x; }
...
</PRE>
<P>
<A NAME=6757> </A>
The two member functions named <tt> Datum</tt> are <em> constructor
<A NAME=6759> </A>
functions
</em> for the <tt> Datum</tt> class. A constructor has the same
name as the class to which it applies and, if defined, is called
whenever an object of the appropriate type is created. Constructor
functions are used to perform initialization and can be overloaded.
<P>
The function <tt> test</tt> in Program <A HREF="node52.html#progccex2" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node52.html#progccex2">5.1</A> creates and uses
three <tt> Datum</tt> objects, two of which are declared in the first two lines
in the function body.
Notice that the class name <tt> Datum</tt>
can be used directly; in C we would have to write <tt> struct Datum</tt>.
In the third line, the <tt> new</tt> operator is used to allocate the third
<tt> Datum</tt> object.
<P>
Because constructors have been defined for <tt> Datum</tt>, they will be
called whenever <tt> Datum</tt> objects are created. The constructor with no
arguments, called a <em> default constructor</em>, is called when <tt>
<A NAME=6771> </A>
a_datum</tt> is created, thereby initializing the field <tt> x</tt> of <tt>
a_datum</tt> to zero. The declaration of <tt> another_datum</tt> and the
<tt> new</tt> operator both specify an integer argument and hence use the
second constructor, thereby initializing the variable <tt> x</tt> to 23 in
these two cases.
<P>
Recall that in C, the fields of a structure are accessed by using the
dot operator (<tt> struct.fieldname</tt>), while the fields of a structure
accessible via a pointer are accessed with the arrow operator (<tt>
structptr->fieldname</tt>). As illustrated in the function <tt> test</tt>,
these same mechanisms can be used to refer to the member functions of
a C++
class.
<P>
The C++
class mechanism also supports <em> protection</em>. Members of
a C++
class can be designated as being either <tt> public</tt> or <tt>
private</tt>. A <tt> public</tt> class member can be used without restriction
<A NAME=6784> </A>
by any program that has a reference to an instance of a class. Public
<A NAME=6785> </A>
data members can be read or written, and public member functions may be
called. In contrast, private members can be accessed only from within
the class object. Private data members can be accessed only by a
class member function, and private member functions can be called only
from within another member function of the class. For example, the
variable <tt> x</tt> in the <tt> Datum</tt> class is a private variable and
hence can be accessed by the member function <tt> get_x</tt> but cannot
be referenced directly as <tt> a_datum.x</tt>.
<P>
<H2><A NAME=SECTION03213000000000000000>5.1.3 Inheritance</A></H2>
<P>
<A NAME=secccii> </A>
<P>
The final C++
feature described here is inheritance. As in C, a
<A NAME=6792> </A>
class or structure can be included as a member of another class, hence
<A NAME=6793> </A>
defining a <em> has-a</em> relationship between the two classes. In C++
,
inheritance is used to create an alternative relationship between
classes, an <em> is-a</em> relationship. If a class D inherits from class
B, then all public members of B are also members of D. We say that D
is derived from B, and that D is a derived class while B is a base
class. D includes all public members of B and may also include
additional members, which are defined in the usual way. We can view D
as being a specialized version of a B, hence the <em> is-a</em>
relationship.
<P>
Program <A HREF="node52.html#progcci" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node52.html#progcci">5.2</A> illustrates the use of inheritance. The
syntax for inheritance is to specify a list of base classes after the
derived class name. The base class list is separated from the derived
class name by a colon. The keywords <tt> public</tt> and <tt> private</tt>
<A NAME=6800> </A>
are associated with the base class names to specify whether the
inherited members are to be public or private members of the derived
class.
<P>
Members of the base class can be redefined in the derived class. For
example, in Program <A HREF="node52.html#progcci" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node52.html#progcci">5.2</A> class D redefines <tt> func2</tt>.
When <tt> func2</tt> is called from an object of type B, we access the
version of <tt> func2</tt> defined in B. If <tt> func2</tt> is called from an
object of type D, we get the version of <tt> func2</tt> defined in D.
<P>
<P><A NAME=progcci> </A><IMG BORDER=0 ALIGN=BOTTOM ALT="" SRC="img832.gif" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/img832.gif"><P>
<P>
In some situations, we may want a base class to call functions that
are defined in a derived class. This facility is supported by a
C++
mechanism called <em> virtual functions.</em> A function declared
<A NAME=6828> </A>
virtual in a base class can be defined in a derived class. This
<A NAME=6829> </A>
feature, which allows a programmer to specialize a generic base class
for a specific application, is used in Section <A HREF="node59.html#secccvf" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node59.html#secccvf">5.8.2</A> to
build a reusable parallel library.
<P>
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<B> Next:</B> <A NAME=tex2html2568 HREF="node53.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node53.html">5.2 CC++
Introduction</A>
<B>Up:</B> <A NAME=tex2html2566 HREF="node51.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node51.html">5 Compositional C++
</A>
<B> Previous:</B> <A NAME=tex2html2560 HREF="node51.html" tppabs="http://www.dit.hcmut.edu.vn/books/system/par_anl/node51.html">5 Compositional C++
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