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Other
keywords are used in the declaration of a class.
Two of the most important are public and private.
All
members of a class—data and methods—are private
by default. Private members can be accessed only
within methods of the class itself. Public members
can be accessed through any object of the class.
This distinction is both important and confusing.
To make it a bit clearer, consider an example from
earlier in this unit:
class
Cat
{
unsigned
int itsAge;
unsigned
int itsWeight;
Meow();
};
In
this declaration, itsAge, itsWeight, and Meow()
are all private, because all members of a class
are private by default. This means that unless you
specify otherwise, they are private.
However,
if you write
Cat
Boots;
Boots.itsAge=5;
// error! can’t access private data!
the
compiler flags this as an error. In effect, you’ve
said to the compiler, "I’ll access itsAge, itsWeight,
and Meow() only from within member functions of
the Cat class." Yet here you’ve accessed the itsAge
member variable of the Boots object from outside
a Cat method. Just because Boots is an object of
class Cat, that doesn’t mean that you can access
the parts of Boots that are private.
This
is a source of endless confusion to new C++ programmers.
I can almost hear you yelling, "Hey! I just said
Boots is a cat. Why can’t Boots access his own age?"
The answer is that Boots can, but you can’t. Boots,
in his own methods, can access all his parts—public
and private. Even though you’ve created a Cat, that
doesn’t mean that you can see or change the parts
of it that are private.
The
way to use Cat so that you can access the data members
is
class
Cat
{
public:
unsigned
int itsAge;
unsigned
int itsWeight;
Meow();
};
Now
itsAge, itsWeight, and Meow() are all public. Boots.itsAge=5
compiles without problems.
Listing
6.1 shows the declaration of a Cat class with public
member variables.
Listing
6.1. Accessing the public members of a simple
class.
1:
// Demonstrates declaration of a class and
2:
// definition of an object of the class,
3:
4:
#include <iostream.h> // for cout
5:
6:
class Cat // declare the class object
7:
{
8:
public: // members which follow are public
9:
int itsAge;
10:
int itsWeight;
11:
};
12:
13:
14:
void main()
15:
{
16:
Cat Frisky;
17:
Frisky.itsAge = 5; // assign to the member variable
18:
cout << "Frisky is a cat who is " ;
19:
cout << Frisky.itsAge << " years old.\n";
20:
Output:
Frisky is a cat who is 5 years old.
Analysis: Line 6 contains the keyword
class. This tells the compiler that what follows
is a declaration. The name of the new class comes
after the keyword class. In this case, it is Cat.
The body of the declaration begins with the opening
brace in line 7 and ends with a closing brace and
a semicolon in line 11. Line 8 contains the keyword
public, which indicates that everything that follows
is public until the keyword private or the end of
the class declaration.
Lines
9 and 10 contain the declarations of the class members
itsAge and itsWeight.
Line
14 begins the main function of the program. Frisky
is defined in line 16 as an instance of a Cat—that
is, as a Cat object. Frisky’s age is set in line
17 to 5. In lines 18 and 19, the itsAge member variable
is used to print out a message about Frisky.
NOTE: Try commenting out line 8 and
try to recompile. You will receive an error on line
17 because itsAge will no longer have public access.
The default for classes is private access.
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