The C++ class and the object

The definition of the C++ class

Defining a class is essentially a blueprint for defining a data type. This does not actually define any data, but it defines what the name of the class means, that is, it defines what the object of the class includes and what can be done on that object.

The class definition begins with the keyword class, followed by the name of the class. T he body of the class is contained in a pair of parentheses. T he class must be defined with a sign or a list of declarations. For example, we use the keyword class to define a Box data type, as follows:

class Box
{
   public:
      double length;   // Length of a box
      double breadth;  // Breadth of a box
      double height;   // Height of a box
};

The keyword public determines the access properties of class members. W ithin the scope of a class object, public members are accessible outside the class. You can also specify that the members of the class are private or protected, which we'll cover later.

Define the C++ object

The class provides a blueprint for the object, so basically, the object is created from the class. D eclares the object of the class, just as it declares a variable of the base type. The following statement declares two objects of the class Box:

Box Box1;          // 声明 Box1，类型为 Box
Box Box2;          // 声明 Box2，类型为 Box

Objects Box1 and Box2 have their own data members.

Access data members

Public data members of class objects can be accessed using the direct member access operator (.). To better understand these concepts, let's try the following example:

#include <iostream>

using namespace std;

class Box
{
   public:
      double length;   // 长度
      double breadth;  // 宽度
      double height;   // 高度
};

int main( )
{
   Box Box1;        // 声明 Box1，类型为 Box
   Box Box2;        // 声明 Box2，类型为 Box
   double volume = 0.0;     // 用于存储体积
 
   // box 1 详述
   Box1.height = 5.0; 
   Box1.length = 6.0; 
   Box1.breadth = 7.0;

   // box 2 详述
   Box2.height = 10.0;
   Box2.length = 12.0;
   Box2.breadth = 13.0;

   // box 1 的体积
   volume = Box1.height * Box1.length * Box1.breadth;
   cout << "Box1 的体积：" << volume <<endl;

   // box 2 的体积
   volume = Box2.height * Box2.length * Box2.breadth;
   cout << "Box2 的体积：" << volume <<endl;
   return 0;
}

When the above code is compiled and executed, it produces the following results:

Box1 的体积：210
Box2 的体积：1560

It is important to note that private members and protected members cannot use the direct member access operator (.) for direct access. We'll learn how to access private members and protected members in a later tutorial.

Classes and objects are detailed

So far, we've got a basic understanding of the classes and objects in C+ . There are other concepts related to C++ classes and objects in the following list, which you can learn by clicking on the appropriate links.