[classes and objects] explain classes and objects in simple terms

1. What is an object ？

​   Object is an abstract concept , It means anything that exists . Everything in the world can be an object , In the real world , A thing that can be seen everywhere is an object , Object is the existence of things Entity , A specific person is an object , A specific book can also be an object ……

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2. What is a class? ？

​   Classes are encapsulating objects “ attribute ” and “ Behavior ” The carrier of . On the other hand , Having the same properties and behavior A class Entities are called classes . for instance ：

（ Picture from the Internet ）

​   The range of classes can be large or small , As long as they have the same attributes and behaviors, they can be classified into one class . If we will Instantiation , You can get a specific object .

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3. How to define a class ？

​   Classes are defined in two ways , Early use struct To define a class , Later introduced keywords class To define a class , Now we are still used to using class To define a class .

① use class Defining classes

class className
{
     
	//  The class body ： It consists of member functions and member variables 
};  //  Be sure to pay attention to the semicolon behind 

② use struct Defining classes ：

struct dog
{
     
    // action 
    void eat() 
    {
     
        cout << "The dog is eating!" << endl;
    }
    // attribute 
    int _age;
    char _name[10];
};

C++ The structure in is upgraded to class ：

• The structure name can be directly used as the type

  struct ListNode
  {
         
   	int val;
     	ListNode* next; // C++ There is no need to add struct
  };
  

• “ Structure ” Function can be defined in

• Of course, for compatibility C Language ,C The operation of structure in language is the same

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4. encapsulation

​   stay C In language , Data and methods are separated . When data and method are separated , Although users have higher degrees of freedom , But the Literacy Requirements for users are also higher . An example of image ： If there is no fence in the scenic spot , There is no fixed access , Then people can go in and out at will , Vandalism of scenic spots ; Corresponding to our program , There is a risk of data being tampered .

​   We can see ,C++ Classes in put data and methods together , adopt Access qualifier To protect some member variables and member functions , Thus ensuring the integrity of the internal data structure of the class , Prevent external operations from interfering with internal data .

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5. Access qualifier

​ There are three types of access qualifiers , There are several points to pay attention to ：

• public Decorated members can be accessed directly outside the class
• protected and private Decorated members cannot be directly accessed outside the class （ In package protected and private The function of is similar ）
• There can be multiple access qualifiers in a class , The access scope starts from where the access qualifier appears until the next access qualifier appears
• class The default access rights of are private,struct by public( because struct To be compatible with C)

class classname
{
     
public:
    // ……  The access rights of member functions are usually public

private:
    // ……  The access rights of member variables are usually private
};

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6. Definition domain of class

​   Because the class is a whole , When finding variables, you will find them in the whole class , So member functions can be defined anywhere .

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7. Definition of member function

There are also two ways to define member functions ：

① Defined in class

class student
{
     
public:
    void init(int age, int score)
    {
     
        _age = age;
        _score = score;
    }

private:
    int _age;
    int _score;
};

• The functions defined in the class are all inline functions by default . This also points the way for us to write code , The functions defined in the class are suitable for short functions
• Member variables are customarily preceded by “_”, otherwise init The parameter names of functions are awkward

② Define... Outside the class

class student
{
     
public:
    void init(int age, int score);

private:
    int _age;
    int _score;
};

void student::init(int age, int score)
{
     
    _age = age;
    _score = score;
}

• Put declaration in class , The definition is placed outside the class

• To illustrate that this function is a member function of a class , You need to add the class name and scope decomposition operator before the function name

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8. Instantiation

//  For example, use the above ListNode Class to create objects ;
int main()
{
     
    ListNode node;
}

reflection ： Whether member variables in a class are defined or declared ？

【 answer 】 Whether to define or declare depends on whether to open up space . When creating classes and opening up space for member variables , So it's just a statement , It is only defined when instantiating

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9. How to calculate the size of an object ?

  There are both member variables and member functions in the object . For member variables , Each object must be independent and different from each other , However, member functions only need to store a copy in a fixed place . The specific storage mode is shown in the figure below . When calculating the size of member variables, you still need to follow the rules of memory alignment .

  Under disassembly, we can observe , When calling member functions for multiple objects created by the same class ,call The function addresses corresponding to the instructions are the same .

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10. this Introduction of pointer

【 problem 】 Member functions are stored in the common code area , So there is no distinction between different objects in the function body , So how does a function know which object it operates on ？

【 resolvent 】C++ Introduce keywords this The pointer . Every The static Member functions have a hidden parameter this The pointer , Calling The static A pointer parameter will also be passed in by default when the member function , The function of this pointer is to point to the current object .
 
​ Let's illustrate with the following example ：

class Date
{
     
public :
    void Display ()
    {
     
        cout <<_year<< "-" <<_month << "-"<< _day <<endl;
    }
    void SetDate(int year , int month , int day)
    {
     
        _year = year;
        _month = month;
        _day = day;
    }
private :
    int _year ; //  year 
    int _month ; //  month 
    int _day ; //  Japan 
};

Feature summary ：

• ① this The function of pointers is to ensure that each object has its own data members , But share the code that processes this data

• ② Static member functions （ The function name is prefixed with static） Shared by all objects , So there was no this The pointer

• ③ this Pointers are automatically passed by the compiler , Cannot be written in formal parameters , You can't pass parameters manually . but this Pointers can be used

  

  

  It can also prove that this The pointer Point to current object