Private permission problem in C++ copy constructor

introduction

In C++, a copy constructor is a special member function used to create a copy of an object. A copy constructor is usually defined as a public member function so that other objects can use it to create copies. However, some people may have doubts about whether the copy constructor can access private member variables. This article will discuss in detail the Private permission issue in the C++ copy constructor.

analyze

1. The concept of access rights: In C++, access rights are used to control the accessibility of class members. C++ provides three access rights: public (public), private (private) and protected (protected). Public members can be accessed inside and outside the class, private members can only be accessed inside the class, and protected members can be accessed inside the class and in derived classes.

2. The role of the copy constructor: The copy constructor is a special member function used to create a copy of an object. It is usually used in the following situations:

   • when an object is initialized as a copy of another object of the same type;
   • when an object is passed by value as a function parameter;
   • when an object is returned as a function return value;

3. Access rights of the copy constructor: Although the copy constructor is a public member function, it can still access the private member variables of the class. This is because the copy constructor is a member function of a class and it has access to all members of that class, including private members.

4. Sample code:

MyClass {
public:
    MyClass(int value) : privateVar(value) {}

    // 拷贝构造函数
    MyClass(const MyClass& other) {
        privateVar = other.privateVar;
    }
    
private:
    int privateVar;
};

In the above example, the copy constructor can access the private member variable privateVar and assign it to the newly created object.

5. Restriction of access rights: It should be noted that although the copy constructor can access private member variables, it can only be used inside the class. External code cannot directly access private member variables, including objects created through copy constructors.

Summarize

Member functions of C++ classes can access private member variables of instance objects of the same type because member functions are part of the class and they are regarded as the internal code of the class. Inside a class, all member functions can access the private members of that class.