[C++] `this` pointer in C++: in-depth exploration and application

1 Introduction:

    thisPointers are a key concept in C++ that play an important role in object-oriented programming. It is a pointer to the current object, and member variables and member functions of the object can be accessed through thisthe pointer . Understanding and using thispointers is critical to writing high-quality C++ code.

        The purpose of this article is to provide an in-depth look at the use of thispointers , including their basic understanding, practical applications, limitations of use, and their role in different programming scenarios. We will start from the basic concepts and gradually deepen, including the use of thispointers in member functions, solving variable naming conflicts, implementing chain calls, the role of inheritance and polymorphism, constspecial behaviors in member functions, and multi-threaded programming. application etc. Through this article, readers will have a more comprehensive understanding thisof pointers , and be able to use it correctly and effectively to improve their own C++ code.

An overview of the structure of the article:

This article will introduce the relevant knowledge and application of thispointers :

1. introduction

   • Introduction Definition and Importance of thisPointers
   • Outline the purpose and structure of the paper

2. thisBasics of Pointers

   • Explain the definition and basic understanding of thispointers
   • Introduce how to use thispointers to access member variables and member functions

3. thisApplication of pointers in practical programming

   • Explore the role of thispointers in resolving variable naming conflicts
   • Discuss how to use thispointers to implement chained calls
   • Explain the role of thispointers in copy constructors and assignment operators
4. thisThe role of pointers in inheritance and polymorphism
   • Explain the behavior of thispointers in virtual functions and overrides
   • Explain in detail how to implement dynamic binding through thispointers
5. thispointers and constmember functions
   • Emphasis on the use of constmember functions to ensure the invariance of object state
   • constExplain thisthe type and meaning of pointers in member functions
6. thisThe use of pointers in multithreaded programming
   • Emphasis on using thispointers to deal with multithreaded synchronization and data race issues
   • Explain the behavior of thispointers in a multi-threaded environment
7. thisPointer Considerations and Common Pitfalls
   • Emphasis on avoiding returning thispointers
   • Provides restrictions on the use of thispointers in constructors and destructors
8. Summarize
   • thisEvaluate the value of pointers in C++ programming
   • Review the key knowledge points and usage skills of thispointers

   • Through the above structure, we will comprehensively and systematically discuss the relevant content of thispointers to help readers better understand and apply this important concept.

Table of contents

1 Introduction:

2 Basics of the `this` pointer

(1) Definition and basic understanding of this pointer

3 Application of `this` pointer in actual programming

(1) Resolve variable naming conflicts

(2) Chain call

 (3) copy constructor and assignment operator

4 The role of the `this` pointer in inheritance and polymorphism

(1) Realize dynamic binding through this pointer

(2) Behavior of this pointer in virtual functions and overrides

5 `this` pointer and `const` member function

(1) Type and meaning of this pointer in const member function

(2) Use const member functions to ensure the invariance of the object state

6 Use of the `this` pointer in multi-threaded programming

(1) Behavior of this pointer in a multi-threaded environment

(2) Use this pointer to handle multi-thread synchronization and data competition

7 Caveats and common pitfalls of the `this` pointer

(1) Restrictions on the use of this pointer in constructors and destructors

(2) Avoid returning the this pointer of the object to be destructed

8 Summary:

---

2 Basics of the `this` pointer

        In object-oriented programming in C++, thisa pointer is a special pointer that points to the object that called the member function. Inside a non-static member function, you can use thispointers to access members of the calling object. Essentially, thisthe pointer is provided implicitly by the compiler, we don't need to define it, but we can use it inside member functions.

(1) thisDefinition and basic understanding of pointers

        In C++, thisit is a pointer to the current object, thisthrough which access all members of the current object. thisActually a pointer to the current class type, eg, for a member function Boxof , thisa pointer of Box*type .

class Box {
    int length;  // 定义私有的整型变量length
public:
    void setLength(int length) {
        this->length = length;  // this指针指向当前对象，并将传入的整数值赋给length成员变量
    }
};

(2) thisThe use of pointers in member functions, and how to thisaccess

    thisPointers can be used in all non-static member functions of a class, including constructors and destructors. We can use thispointers to access member variables and member functions.

class Box {
    int length;  // 定义私有的整型成员变量length
public:
    Box(int length) {  
        this->length = length;  // 构造函数，初始化成员变量length
    }

    int getLength() {
        return this->length;  // 访问成员变量length
    }

    void display() {
        std::cout << "Length: " << this->getLength() << std::endl;  // 类的成员函数，访问成员变量length和其他函数
    }
};

        In the above code, setLengthwe used thisthe pointer in the function to access the member variable length. In displaythe function , we thiscalled the getLengthmember .

        Remember, we can use thispointers , because only at this time, thisthe pointer has the object instance being called to point to.

3 Application of `this` pointer in actual programming

  thisPointers are widely used in programming practice. It can help us resolve variable naming conflicts, implement chain calls, and play a key role in copy constructors and assignment operators.

(1) Resolve variable naming conflicts

        In a member function of a class, if the name of the formal parameter is the same as the name of a member variable of the class, thispointers disambiguate.

class Box {
    int length;  // 定义私有的整型成员变量length
public:
    void setLength(int length) {
        this->length = length;  // 使用this指针访问成员变量length并将参数length的值赋给它
    }
};

        In this example, thispointers are used to lengthresolve lengthnaming conflicts between formal member variables.

(2) Chain call

        Chaining is a programming technique that makes code more compact and readable. It *thisallows multiple member function calls to be made consecutively on the same line by returning in each member function.

class Box {
    int length, width;  // 定义私有的整型成员变量length和width
public:
    Box& setLength(int length) {  // 返回一个指向当前对象的引用
        this->length = length;  // 将传递的参数 length 赋给成员变量 length
        return *this;  // 返回指向当前对象的引用
    }

    Box& setWidth(int width) {  // 返回一个指向当前对象的引用
        this->width = width;  // 将传递的参数 width 赋给成员变量 width
        return *this;  // 返回指向当前对象的引用
    }

    void display() {  // 定义成员函数 display
        std::cout << "Length: " << length << ", Width: " << width << std::endl;  // 输出成员变量 length 和 width 的值
    }
};

// 使用示例
Box b;
b.setLength(10).setWidth(5).display();  // 链式调用 setLength, setWidth, display 函数显示结果

 (3) copy constructor and assignment operator

thisPointers also play an important role in copy constructors and assignment operators. In these functions, we usually need to check whether the object passed in is the current object (ie, whether it is self-assigned). If so, you should avoid doing self-assignments that could lead to errors.

class Box {
    int* data;  // 定义私有指针成员变量 data

public:
    // 赋值运算符重载函数
    Box& operator=(const Box& other) {
        if (this != &other) {  // 防止自赋值的情况
            delete[] data;  // 释放旧内存
            data = new int[10];  // 重新分配内存
            std::copy(other.data, other.data + 10, data);  // 拷贝数据
        }
        return *this;  // 返回一个指向当前对象的引用
    }
};

        In this example, we first check thisif is equal &other, if it is, then this is self assignment and we should avoid performing operations that may destroy the state of the object. This is a common use of thispointers in copy constructors and assignment operators.

4 The role of the `this` pointer in inheritance and polymorphism

        In C++, thispointers play a very important role in dealing with inheritance and polymorphism issues. It plays an important role in dynamic binding, virtual functions and overriding etc.

(1) Realize dynamic binding through thispointers

        Dynamic binding is one of the key mechanisms of polymorphism, which makes it possible to call the corresponding member function according to the actual type of the object at runtime. thisPointers play a key role in this process.

class Base {
public:
    virtual void print() const { // 定义虚函数 print() ，用来打印 Base 类的信息
        std::cout << "Base::print()" << std::endl;
    }
};

class Derived : public Base {  // Derived 类从 Base 类公有继承
public:
    void print() const override { // 重写基类的虚函数 print() 用来打印 Derived 类的信息
        std::cout << "Derived::print()" << std::endl;
    }
};

void callPrint(const Base* base) {  // 定义一个函数，参数类型为指向 Base 类的常指针
    base->print();  // Dynamic binding 动态绑定，通过指针调用 Fprint() 函数
}

// 使用示例
Derived d;  // 创建 Derived 对象
callPrint(&d);  // 通过 callPrint 函数打印对象 d 的信息，输出 "Derived::print()"

        In the above code, we have a base class Baseand a derived class Derived, both of which have a printmember function. In callPrintthe function , we call the function through thisthe pointer ( ), at which point dynamic binding occurs.baseprint

(2) thisBehavior of pointers in virtual functions and overrides

        In virtual functions and overrides, thisthe pointer points to the object that was originally used to call the member function. If we override the virtual function of the base class in the derived class, then thisthe pointer type in this virtual function is still the pointer type of the base class, but it actually points to the object of the derived class.

class Base {
public:
    virtual void print() const {  // 定义虚函数 print() 打印基类的信息
        std::cout << "This is a Base object." << std::endl;
    }
};

class Derived : public Base {
public:
    void print() const override {  // 重写基类的虚函数 print() ，用来打印派生类的信息
        Base::print(); // 首先调用基类的 print() 函数打印基类信息
        std::cout << "This is a Derived object." << std::endl;  // 再打印派生类信息
    }
};

// 使用示例
Derived d;   // 创建 Derived 对象
d.print();   // 打印 Derived 对象的信息，输出 "This is a Base object." 和 "This is a Derived object."

        In this example, when we dcall printthe function through the derived class object, thisthe pointer points to dthe object . In Derived::printthe function , we first call printthe function of the base class, and then output a line of information. This shows that even in the derived class that overrides the virtual function of the base class, thisthe pointer can still correctly point to the object of the derived class.

5 `this` pointer and `const` member function

        In C++, there is something special about the behavior of thispointers in constmember functions. In this part, we will discuss in thisdetail constthe type and meaning of pointers in member functions, and how to use constmember functions to ensure the invariance of object state.

(1) Types and meanings of pointersconst in member functionsthis

        In a constmember function, thisthe type of the pointer is a pointer to consttype . This means you cannot modify the state of the object through thisthe pointer .

class Box {
    int length;

public:
    int getLength() const { // 常成员函数
        // this->length = 10;  // 错误：不能在常成员函数中修改成员变量
        return length;  // 返回成员变量的值
    }
};

        In this example, getLengtha constmember function. In this function, thisthe type of the pointer is const Box*, so you cannot modify it through thisthe pointer length.

(2) Use constmember functions to ensure the invariance of the object state

  constMember functions are an important mechanism for ensuring the invariance of an object's state. When you declare a member function as const, you are promising that the function will not modify the state of the object. This is very useful for writing stable and reliable code.

class Box {
    int length;

public:
    Box(int length) : length(length) {}  // 构造函数，初始化成员变量 length

    void increaseLength(int increment) {  // 增加盒子长度的函数
        length += increment;  // 修改对象的状态
    }

    int getLength() const {  // 获取盒子长度的函数，常成员函数
        return length;  // 不修改对象的状态，只返回成员变量的值
    }
};

        In this example, increaseLengtha non- constmember function that modifies the state of an object. Rather getLength, constit is a member function that cannot modify the state of the object. When you need to read the state of an object, but don't want to modify it, you should use constmember functions.

6 Use of the `this` pointer in multi-threaded programming

        Multithreaded programming is an important part of modern computing, which allows multiple tasks to be executed simultaneously in a single program. There is something special about the behavior and use of pointers in this environment this, especially when dealing with issues of multithread synchronization and data races.

(1) thisBehavior of pointers in a multi-threaded environment

        In a multi-threaded environment, each thread has its own independent stack space, so in each thread, thisthe value of the pointer is independent. In other words, thispointers can only be passed within the same thread, not across threads.

        Suppose we have a class Box, and we create Boxobjects , then in each thread, thisthe pointer will point to its respective created Boxobject .

(2) Use thispointers to handle multi-thread synchronization and data competition

        In multithreaded programming, thispointers are often used to deal with multithreaded synchronization and data race issues. For example, we may need to use thisa pointer to acquire a mutex to protect the class's data members from concurrent access.

#include <mutex>
#include <thread>

class Box {
    int data;
    std::mutex mtx;  // 线程互斥量，用于保护共享数据的访问
public:
    void setData(int value) {
        std::lock_guard<std::mutex> lock(mtx);  // 申请互斥锁
        this->data = value;  // 修改共享数据
    }
};

void threadFunction(Box* box, int value) {
    box->setData(value);  // 更新共享数据
}

// 使用示例
Box b;   // 创建共享对象
std::thread t1(threadFunction, &b, 10);  // 创建第一个线程并传入共享对象的地址以及值
std::thread t2(threadFunction, &b, 20);  // 创建第二个线程并传入共享对象的地址以及值
t1.join();  // 等待第一个线程执行完毕
t2.join();  // 等待第二个线程执行完毕

        In this example, setDatathe member function uses thisthe pointer to acquire a mutex to protect datathe member variable. When we call setDatathe function time, only one thread can access datathe member variable due to the existence of the mutex, which avoids the problem of data competition.

7 Caveats and common pitfalls of the `this` pointer

        Although thispointers are a powerful tool, there are some pitfalls and limitations to be aware of when using them, especially with respect to constructors, destructors, and returning thispointers to objects that are about to be destructed.

(1) thisRestrictions on the use of pointers in constructors and destructors

        Using thispointers requires special care. In a constructor, using thisa pointer . Also, in destructors, using thispointers .

class Box {
    int* data;

public:
    Box() {
        data = new int[10];
        // 避免使用 'this' 进行重要操作，因为对象没有完全构造完成
    }

    ~Box() {
        delete[] data;
        // 对象正在被析构，进一步使用 'this' 可能会导致未定义行为
    }
};

(2) Avoid returning thispointers

        A common pitfall is returning thispointers , especially when the member function is a member function of a temporary object. After the function returns, the temporary object is destructed, and the returned thispointer becomes a dangling pointer, which leads to undefined behavior.

class Box {
    int data;

public:
    Box(int value) : data(value) {}

    Box* getDataPtr() {
        return this;
    }
};

Box* badFunc() {
    return Box(10).getDataPtr();  // 返回指向已经被析构的对象的指针
}

// 使用示例
Box* p = badFunc();  // 'p' 是一个悬垂指针（dangling pointer）

        In the above code, badFuncthe function returns a thispointer to , which causes pbecomes a dangling pointer. In order to avoid this situation, we should try to avoid returning thispointers , or ensure thisthat the object pointed to by the returned pointer still exists after the function returns.

8 Summary:

        This article delves into the importance and application of thispointers . The following is a review of key knowledge points and usage skills of thispointers , as well as thisan evaluation of the value of pointers in C++ programming.

• thisA pointer is a special pointer to the current object. Inside a non-static member function, thisa pointer to access members of the calling object.
• thisThe use of pointers in member functions is very flexible, and member variables can be accessed this->memberthrough , or this->function()member functions can be called through .
• When resolving variable naming conflicts, thispointers can help remove ambiguity and make code clearer and easier to understand.
• thisPointers play an important role in chain calls. By returning *this, multiple member functions can be called continuously in the same line.
• In copy constructors and assignment operators, thispointers are used to handle self-assignment situations, avoiding operations that may lead to errors.
• In inheritance and polymorphism, thispointers enable dynamic binding, ensuring that the correct function implementation is called.
• In virtual functions and overrides, thisthe pointer remains accurate across virtual function calls, pointing to the object that was originally used to call the member function.
• In constthe member function, thisthe type of the pointer becomes const T*, which is used to ensure the invariance of the object state.
• In a multi-threaded environment, each thread has its own thispointer , so thispointers cannot be passed across threads. In multithreaded programming, thispointers are often used to deal with multithreaded synchronization and data race issues.
• There are restrictions on the use of thispointers . In constructors, thispointers cannot be used to access uninitialized member variables; in destructors, using thispointers may result in undefined behavior.
• Avoid returning thispointers to to avoid problems with dangling pointers.

overall evaluation:

   thisPointers are a powerful feature in C++ that provide us with the ability to access the current object and play an important role in many programming scenarios. By properly understanding and using thispointers , we can write cleaner and more efficient code.