Classes and objects -- object model and this pointer

.1 The object model

Member variables are stored separately from member functions .
Memory occupied by empty objects 1 byte , Only non static member variables belong to objects of class

.2this The pointer

Because multiple objects of the same type will share a piece of code , In order to distinguish between ,C++ Provide this The pointer Point to the object to which the called member function belongs （ There is no need to define direct use ）

1. When a formal parameter has the same name as a member variable , You can use this Pointer discrimination

class Person {
    
public:

	void test(int age) {
    
		this ->age = age;
	}
	int age;
};

2. When the object itself is returned in a member function in a class , Use return *this
   

.3 Null pointer to access member function

this Error caused by null pointer , To ensure robustness, you need to verify

class Person {
    
public:
	int age;

	void show() {
    
		if (this == NULL) {
    
			return;					// testing this Is the pointer null 
		}
		cout << age << endl;		// Equivalent to calling this->age 
	}
};

void test() {
    
	Person* p1 = NULL;

	p1->show();
}

.4 const Modifying Member Functions

Constant function ： Add... After the member function const

1. Member properties cannot be modified in a constant function , Modification is this Point to , Make it not modifiable
2. Prefix the member attribute declaration with the keyword mutable after , Can only be modified in a constant function

Regular object ：const Class name p;

1. Constant objects cannot modify member properties , Unless added mutable
2. A constant object can only call a constant function

class Person {
    
public:
	mutable int age ;

	//this The essence of pointer is pointer constant , The point of the pointer cannot be modified 
	void show() const
	{
    
		this->age = 10;
	}
};