DATA
ABSTRACTION:
·
Data abstraction refers to, providing only
essential information to the outside world and hiding their background details,
i.e., to represent the needed information in program without presenting the
details.
·
Data abstraction is a programming (and design)
technique that relies on the separation of interface and implementation.
·
Let's take one real life example of a TV, which you
can turn on and off, change the channel, adjust the volume, and add external
components such as speakers, VCRs, and DVD players, BUT you do not know its
internal details, that is, you do not know how it receives signals over the air
or through a cable, how it translates them, and finally displays them on the
screen.
·
Thus, we can say a television clearly separates its
internal implementation from its external interface and you can play with its
interfaces like the power button, channel changer, and volume control without
having zero knowledge of its internals.
·
Now, if we talk in terms of C++ Programming, C++
classes provides great level of data
abstraction. They provide sufficient public methods to the outside world to
play with the functionality of the
object and to manipulate object data, i.e., state without actually knowing how
class has been implemented internally.
·
In C++, we use classes
to define our own abstract data types (ADT). You can use the cout object of class ostream to stream data to standard
output like this:
#include
<iostream.h>
int main(
)
{
cout
<< "Hello C++" <<endl;
return 0;
}
·
Here, you don't need to understand how cout displays the text on the user's
screen. You need to only know the public interface and the underlying
implementation of cout is free to change.
Access Labels Enforce Abstraction:
·
In C++, we use access labels to define the abstract
interface to the class. A class may contain zero or more access labels:
·
Members defined with a public label are accessible
to all parts of the program. The data-abstraction view of a type is defined by
its public members.
·
Members defined with a private label are not
accessible to code that uses the class. The private sections hide the
implementation from code that uses the type.
·
There are no restrictions on how often an access
label may appear. Each access label
specifies
the access level of the succeeding member definitions. The specified access
level remains in effect until the next access label is encountered or
theclosing right brace of the class body is seen.
Benefits of Data Abstraction:
·
Data abstraction provides two important advantages:
·
Class internals are protected from inadvertent
user-level errors, which might corrupt the state of the object.
·
The class implementation may evolve over time in
response to changing requirements or bug reports without requiring change in
user-level code.
·
By defining data members only in the private
section of the class, the class author is free to make changes in the data. If
the implementation changes, only the class code needs to be examined to see
what affect the change may have. If data are public, then any function that
directly accesses the data members of the old representation might be broken.
Data Abstraction Example:
·
Any C++ program where you implement a class with
public and private members is an example of data abstraction. Consider the
following example:
#include
<iostream.h> using namespace std;
class
Adder
{
public:
// constructor
Adder(int i = 0)
{
total =
i;
}
// interface
to outside world
// void
addNum(int number)
{
total +=
number;
}
// interface
to outside world
int getTotal()
{
return
total;
};
private:
// hidden
data from outside world int total;
};
int main(
)
{
Adder a;
a.addNum(10); a.addNum(20); a.addNum(30);
cout
<< "Total " << a.getTotal() <<endl; return 0;
}
When the
above code is compiled and executed, it produces the following result: Total 60
Above
class adds numbers together, and returns the sum. The public members addNum and getTotal are the interfaces to the outside world and a user needs
to know them to use the class. The
private member total is something
that the user doesn't need to know about, but is needed for the class to
operate properly.
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