class

Example by: Ira Oldham, References: Savitch eighth edition, section 10.2; Gaddis seventh edition, section 13.2

The keywords struct and class differ only that struct defaults to public, while class defaults to private. Of course, you can change between private and public in either one with public: and private:

1  class Sum
2  {
3    int a;
4    int b;
5  public:
6    int getA() {return a;}
7    int getB() {return b;}
8    void putA(int value) {a = value;}
9    void putB(int value) {b = value;}
10   int sum();
11 };
12 int main(void)
13 {
14   Sum x;
15   int result;
16   x.putA(3);
17   x.putB(4);
18   result = x.sum();
19   cout << "x.a: " << x.getA() << endl;
20   cout << "x.b: " << x.getB() << endl;
21   cout << "sum: " << x.sum()  << endl;
22   return 0;
23 }
24 int Sum::sum()
25 {
26   return a + b;
27 }

x.a: 3
x.b: 4
sum: 7

• line 1 class is coded, rather than struct. Now we can use the Object Orientated terminology and say that this is the definition of the class Sum.
• lines 3 a class starts in private
• line 5 The public: access specifier is used to change to public
• line 14 Now we can use the Object Oriented terminology and say that the data in x is an object which is an instance of the class Sum.

A class and a struct are almost the same thing, differing only in the default initial access of public in struct, and private in class. While the are very similar, they are used for different purposes.

When you want to just use public data, use struct. When you want to build a class use class. Important style rules:

• struct is used when you only have public data and NO member functions.
• class is used when you have private data. It is essential to make all the data in a class private. Making all the data private is called encapsulation in Object Oriented programming. Encapsulation is one of the key objectives in Object Oriented programming. Member functions are mostly public.