Sams Teach Yourself C in 21 Days

Working with C++ Classes and Objects 675

BD3

Using a class, you do the following:

time start_time;

time end_time;

This is pretty straightforward. You should note that when you use a class to declare a

new data item, you are actually declaring an object. In other words, in C++ an objectis

simply a declared data item created by using a class. Both start_timeandend_timeare

objects created using the timeclass.

Remember, although a structure is a specialized type of class, there are differences.

Before looking at what is different in Listing B3.3 using classes instead of structures,

first look at the key difference between classes and structures. This difference lies in the

default method to access data in a structure versus the default method in a class.

You use a class to create an object. When you use a class to create an object,

you are instantiatingthe class. Instantiation is simply creating an instance of

a class.

Note

Controlling Access to Data in a Class ..........................................................

Within a class—and hence a structure—you can determine which routines have access to

data by using one of three additional keywords that are a part of C++. These keywords

are:

• public


• private


• protected
  By default, the members of a class are private. This includes both the data members
  and the member functions. For structures, the default access is public. This difference is
  significant.

Public Data

If data is declared as public, it is accessible not only by member functions within the

class or structure, but also by any external source within your program. If you have a

structure such as

struct name {

string firstname;

string lastname;

string formatted_name();

}

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