One 、 The concept of object

It is an entity used to describe objective things in the system , It is a basic unit used to form a system . An object consists of a set of properties and a set of behaviors .

attribute : A data item used to describe the static characteristics of an object .

Behavior : An operation sequence used to describe the dynamic characteristics of an object .

Object is an objective entity in the real world , Its characteristics are :

> Every object has a name ;
> Use attributes （ Or state ) Describe some characteristics of the object ;

> Describe the various behaviors of objects through a set of operations .

State and behavior are the main attributes of objects .
1. The state of an object is also called the static property of the object , It mainly refers to all kinds of information contained in the object , That's the variable . Each individual object has its own internal variables , The values of these variables indicate the state of the object .
2 Object method ( Behavior ) On the one hand, wrap the internal variables of the object , encapsulation , Protect it , So that only the object's own methods can manipulate these internal variables , On the other hand , The method of an object is to interact with the external environment and other objects , Communication interface , The environment of the object and other objects can call the method of the object through this interface , Manipulate the behavior of objects and change the state of objects .

An object is an abstract representation of a real-world entity or concept in computer logic . In particular , An object is a collection of properties and operations with a unique object name and a fixed interface to the outside world , A factor or group of factors used to simulate or affect real-world problems .
 

Two 、 The concept of class

         Ignore the non essential characteristics of things , Only pay attention to the essential characteristics related to the current goal , So as to find out the commonness of things , Divide things of common nature into one class , Come up with an abstract concept .

In object-oriented methods “ class ”

1> A collection obtained by abstracting a group of objects with the same properties and behavior .
2> Provides an abstract description of all objects belonging to this class , It includes two main parts: attribute and behavior .
The relationship between classes and objects :
Such as the relationship between mold and casting , An object belonging to a class is called an instance of that class .

Such as object “ Zhang San ”、“ Li Si ”、…… Have some of the same properties , operation , Abstract it into concrete data , Can be abstracted into a category —— class ： That is, students

A class is an abstraction of an object

Object is an instance of a class , Class materialization

Class is a template for creating objects , It contains the description of the state and operation behavior of the created object ;

An object is an instantiation of a class （ Object variables 、 Object array, etc ）.

class It is an abstraction of objects with the same properties and functions .

Class is an abstract concept , And the object is concrete , Class is just a data type , The object is a variable belonging to this class , Occupy A certain Of Storage unit .

：： scope resolution , Use it to indicate which function or data belongs to which class , Use the full name of the member in the class ：

                                 Class name ：： Member name

3、 ... and 、 The most prominent feature of object-oriented system ：

encapsulation 、 inheritance 、 polymorphism

1> Data abstraction and encapsulation

1. Data abstraction : It is the case analysis of data , Extract the results of their common properties .

         Such as :“ Student “ object , First of all, some examples of students are analyzed , Identify their common or main characteristics ( full name 、 Gender 、 Age 、 Professional and so on ） Ignore differences or minor features that are not interesting （ Difference in appearance 、 Personality differences, etc ).

2. Encapsulation of data
         The data structure and the operations acting on the data structure form an entity , Hide the presentation and operation details of data , The user operates the data through the interface .

         such , The user only knows the operation interface to operate the data , Without knowing how to do it internally and how to represent the encapsulation of data .

2> Data inheritance

Definition : Objects of special classes have all the properties and services of their general classes , It is called the inheritance of a special class from a general class .

        1. Code reusability can be realized and enhanced through inheritance , It is one of the important reasons why object-oriented technology can improve the efficiency of software development .

        2. When creating a new derived class , As long as you specify which base class the derived class is derived from, you can automatically inherit the properties and methods of the base class .
        3. It is not necessary to modify the original code when modifying or expanding the function of the program （ Just add a new code ).

3> Polymorphism of data

Definition : Referring to General Properties or behaviors defined in , By Special class inheritance after , Can have Different data types or behaviors . This makes the same attribute or behavior have different semantics in general classes and their special classes .
         Such as : The addition of numbers : Addition of real numbers
                             Addition of complex numbers
         Such as : max() Function call :( function overloading )
             max(a,b)—— Find the maximum number between two numbers ;

             max(a,b.c)—— Find the maximum number between three numbers ;

Four 、 The relationship between classes and objects

C++ The class is in C Extended from the structure of language . stay C++ in , The type of object is called class , A class is an abstraction of an object , It represents the commonness and characteristics of a group of objects , Object is a concrete instance of a class .

The basic structure of a class
1. name / Mark
2. Data members of class ( Class composition )
3. class ( Class action 、 Behavior )

object : A class variable is an instance of a class , They are called objects .

Structures and classes

In the structure , Members who are not specified as public or private members , The default is Members of the public .

In class , Any member that is not defined as a public member （public) Or protect members (protected) The members of are Private member (private) —— The embodiment of encapsulation .

5、 ... and 、 The type of the class

Is a user-defined data type . If you want to use class types in your program , Must be defined according to actual needs , Or use a designed class .

C++ Define a class , The method is similar to defining a structure type ,

The general form is :

class    Class name

{

        Member list

};

explain
1. Class definition must give the data type declaration of each data member .

 2. When defining a class , The computer does not allocate memory space to data members , Space is allocated only when objects are defined ;
 3. You cannot assign an initial value to a member of a class when it is defined .
 4. Data members can be basic data types , It can also be a complex data type （ Array 、 The pointer 、 quote 、 Structure , It can also be class variables, etc ).

Member visits

Whether data members or function members , Each member of the class has access control properties , have access to Member access qualifier explain :
●public                 ( public )
●private                ( Private )
●protected            ( The protection of )

Member access qualifier

1) Members of the public :public —— The defined members are open , Can be accessed anywhere ( Class and other parts of the program )

public Implements the external interface of the class .

2） Private member :private—— The defined members are hidden , Can only be accessed inside a class , Cannot be accessed elsewhere in the program ;

private Realize the concealment of private members .

3) Protection member : protected—— The defined members are semi open , Can be accessed inside the class , You can also access... In its derived classes , But it cannot be accessed in other parts of the program .

Member access control yes C++ Another important feature of classes and structs .

Add the access label , The more general form of class definition is ：

   class   Class name

     {    public :

       Data member or member function  

    protected:

     Data member or member function

        private :

     Data member or member function

  };

explain :
In the definition of the class ,private、protected、public Can appear any number of times in any order ; But usually private On the front ,protected In the middle ,public Put it at the back .
In actual programming , In order to make the program clear , Each member access qualifier appears only once in the class body , Usually, members with the same access control attributes are written together .
Data members in a class are usually described as private members , To realize data hiding ; Member functions are usually set to public , To access data members via messaging , Protecting members is mainly used to inherit .

class  student

          {  private:

                 int number;

                 char name[15]; 

                 float score;

             public:

                 void display( )                       

                     {  cout<<”number: ”<< number;      

                         cout<<”name: ”<< name;  

                         cout<<”score: ”<< score <<endl;

                      }

         };

Member functions are introduced into class (member function) Or function members : That is, the function becomes data ( class ) A member of the . Class puts the data （ The nature of things ) And the function ( The behavior of things -- operation ) Encapsulate as a whole .
Four data members are declared private , The six function members are described as public ; That is to say, if you operate on four data members from the outside . It can only be done through six public functions , Data is well protected , Not susceptible to side effects . The public function set defines the interface of the class （interface).
Class is a data type , When defined, the system does not allocate storage space for classes , So you can't initialize the data members of a class . Nor can any data member in a class use keywords extern、auto or register Limit its storage type .
Member functions can directly use any member in the class definition , Can handle data members , You can also call function members . Definition of member function :
Previously, only one declaration was made for the member function （ Prototypes of functions ), There is no definition of the function . Function definitions are usually made after the description of the class , The format is as follows :
Return value type class name :: Function name ( Parameter table )
f…]// The body of the function
Where the operator "∵" It is called scope resolution operator (scope resolution operator), It indicates which class the function belongs to .
 

6、 ... and 、 Methods for defining objects

When defining a class , That is, it defines a specific data type . To use a class , You need to instantiate the class , That is, the object that defines this class .

Object definition ：

   1. In a statement （ Definition ） Class while defining objects

   2. First of all （ Definition ） Class redefines the object

   3. Class name does not appear , Define the object directly

1> In a statement （ Definition ） Class while defining objects

class  student 
          {   private:
                  int number; 
                  char name[15];  
                  float score;
               public:
                  void display( )      // Members of the function 
                     {  cout<<”number: ”<< number;       
                        cout<<”name: ”<< name;   
                        cout<<”score: ”<< score <<endl;
                     }
         } stu1, stu2;

2> First of all （ Definition ） Class redefines the object

      Such as ：class  student  stu;

3> Class name does not appear , Define the object directly

class
   {   private:
           int number; 
           char name[15];  
           float score;
       public:
           void display( )      // Members of the function 
               {  cout<<”number: ”<< number;       
                   cout<<”name: ”<< name;   
                   cout<<”score: ”<< score <<endl;
                }
   } stu1,stu2;

class  student 
          {   private:
                  int number; 
                  char name[15];  
                  float score;
               public:
                  void display( )       // Member function definition 
                     {  cout<<”number: ”<< number;       
                        cout<<”name: ”<< name;   
                        cout<<”score: ”<< score <<endl;
                     }
         } ;

class  student
           {   private:
                   int number; 
                   char name[15];  
                   float score;
                public:
                    void display( );       // Member function declaration   
            }  stu1,stu2;
        void student :: display( )     // Member function definition  
           {  cout<<”number: ”<< number;       
               cout<<”name: ”<< name;   
               cout<<”score: ”<< score <<endl;
           }

class  student
           {   private:
                   int number; 
                   char name[15];  
                   float score;
                public:
                    inline void display( );      
            }  stu1,stu2;
        inline void student :: display( )      
           {  cout<<”number: ”<< number;       
               cout<<”name: ”<< name;   
               cout<<”score: ”<< score <<endl;
           }

#include<iostream>
using namespace std;
class time
{
public:
	int hour;
	int minute;
	int sec;
};
int main()
{
	time t1;
	time *p = &t1;
	cin >> p->hour >> p->minute >> p->sec;
	cout << p->hour << ":" << p->minute << ":" << p->sec << endl;
	cin >> (*p).hour >> (*p).minute >> (*p).sec;
	cout << (*p).hour << ":" << (*p).minute << ":" << (*p).sec << endl;
	return 0;

#include<iostream>
using namespace std;
class  time
  {   public:
          int hour; 
          int minute;  
          int sec;
   } ;
int main( )      
  {  time t1;
     time &t2=t1;
     cin>>t2.hour>>t2.minute>>t2.sec;       
     cout<<t2.hour<<”:”<<t2.minute<<”:”<<t2.sec<<endl;   
     return 0;
  }