Object Model and the this Pointer

1.C++Object model and the this pointer

1.1.Member variables and member functions are stored separately

In C++, member variables and member functions within a class are stored separately.

Only non-static member variables belong to objects of the class.

class Person {
public:
	Person() {
		mA = 0;
	}
	//非静态成员变量占对象空间
	int mA;
	//静态成员变量不占对象空间
	static int mB; 
	//函数也不占对象空间，所有函数共享一个函数实例
	void func() {
		cout << "mA:" << this->mA << endl;
	}
	//静态成员函数也不占对象空间
	static void sfunc() {
	}
};

int main() {

	cout << sizeof(Person) << endl;


	return 0;
}

Run result (type size or type name may vary depending on compiler and platform):

4

1.2.Concept of the this pointer

From section 4.3.1, we know that in C++, member variables and member functions are stored separately.

Only one function instance is generated for each non-static member function, which means that multiple objects of the same type share a single piece of code

So the question is: how does this piece of code distinguish which object is calling it?

c++通过提供特殊的对象指针，this指针，解决上述问题。this指针指向被调用的成员函数所属的对象

The `this` pointer is a pointer implicitly contained within every non-static member function.

The this pointer does not need to be defined; it can be used directly.

Purpose of the this pointer:

• When a parameter and a member variable have the same name, the this pointer can be used to distinguish them.
• To return the object itself in a non-static member function of a class, use return *this.

class Person
{
public:

	Person(int age)
	{
		//1、当形参和成员变量同名时，可用this指针来区分
		this->age = age;
	}

	Person& PersonAddPerson(Person p)
	{
		this->age += p.age;
		//返回对象本身
		return *this;
	}

	int age;
};

void test01()
{
	Person p1(10);
	cout << "p1.age = " << p1.age << endl;

	Person p2(10);
	p2.PersonAddPerson(p1).PersonAddPerson(p1).PersonAddPerson(p1);
	cout << "p2.age = " << p2.age << endl;
}

int main() {

	test01();


	return 0;
}

Operation result:

p1.age = 10
p2.age = 40

1.3.Null pointer accesses member function

C++ A null pointer can also be used to call member functions, but you should also be careful whether the this pointer is used.

If the `this` pointer is used, a check must be added to ensure the robustness of the code.

Example:

//空指针访问成员函数
class Person {
public:

	void ShowClassName() {
		cout << "我是Person类!" << endl;
	}

	void ShowPerson() {
		if (this == NULL) {
			return;
		}
		cout << mAge << endl;
	}

public:
	int mAge;
};

void test01()
{
	Person * p = NULL;
	p->ShowClassName(); //空指针，可以调用成员函数
	p->ShowPerson();  //但是如果成员函数中用到了this指针，就不可以了
}

int main() {

	test01();


	return 0;
}

Operation result:

我是Person类!

1.4.const-modified member function

Constant function:

• After adding const to a member function, we call this function aconstant function
• Member attributes cannot be modified within a constant function
• After adding the keyword mutable when declaring a member attribute, it can still be modified in a constant function

Constant object:

• Adding const before declaring an object makes the object a constant object.
• A constant object can only call constant member functions.

Example:

class Person {
public:
	Person() {
		m_A = 0;
		m_B = 0;
	}

	//this指针的本质是一个指针常量，指针的指向不可修改
	//如果想让指针指向的值也不可以修改，需要声明常函数
	void ShowPerson() const {
		//const Type* const pointer;
		//this = NULL; //不能修改指针的指向 Person* const this;
		//this->mA = 100; //但是this指针指向的对象的数据是可以修改的

		//const修饰成员函数，表示指针指向的内存空间的数据不能修改，除了mutable修饰的变量
		this->m_B = 100;
	}

	void MyFunc() const {
		//mA = 10000;
	}

public:
	int m_A;
	mutable int m_B; //可修改 可变的
};

//const修饰对象  常对象
void test01() {

	const Person person; //常量对象  
	cout << person.m_A << endl;
	//person.mA = 100; //常对象不能修改成员变量的值,但是可以访问
	person.m_B = 100; //但是常对象可以修改mutable修饰成员变量

	//常对象访问成员函数
	person.MyFunc(); // 常对象可以调用const成员函数，不可以调用非const成员函数

}

int main() {

	test01();


	return 0;
}

Operation result:

0