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8.3 — Public vs private access specifiers

Access specifiers

Consider the following struct:

In this program, we declare a DateStruct and then we directly access it’s members in order to initialize them. This works because all members of a struct are public members. Public members are members of a struct or class that can be accessed by any function in the program.

On the other hand, consider the following almost-identical class:

If you were to compile this program, you would receive an error. This is because by default, all members of a class are private. Private members are members of a class that can only be accessed by other functions within the class. Because main() is not a member of the Date class, it does not have access to Date’s private members.

Although class members are private by default, we can make them public by using the public keyword:

Because Date’s members are now public, they can be accessed by main().

C++ provides 3 different access specifier keywords: public, private, and protected. We will discuss the protected access specifier when we cover inheritance.

Here is an example of a class that uses all three access specifiers:

Each of the members “acquires” the access level of the previous access specifier. It is common convention to list private members first.

Why would you want to restrict access to class members? Oftentimes you want to declare members that are for “internal class use only”. For example, when writing a string class, it is common to declare a private member named m_nLength that holds the length of the string. If m_nLength were public, anybody could change the length of the string without changing the actual string! This could cause all sorts of bizarre problems. Consequently, the m_nLength is made private so that only functions within the String class can alter m_nLength.

The group of public members of a class are often referred to as a “public interface”. Because only public members can be accessed outside of the class, the public interface defines how programs using the class will interface with the class.

So what’s the difference between a class and a struct? Very little! In C++, a class defaults its members to private. A struct defaults its members to public. That’s it!

8.4 -- Access functions and encapsulation [1]
Index [2]
8.2 -- Classes and class members [3]