Detailed explanation of C++ friend functions: How to declare a friend class?

In C, a friend class allows all member functions of one class to access private members of another class. When declaring a friend class, use the friend class keyword, for example: OuterClass declares the inner class as a friend class: friend class inner class; only the member functions in the inner class can access the private members of OuterClass.

C Detailed explanation of friend function: How to declare a friend class

Preface

In C, friend relationships are a powerful mechanism that allows non-member functions to access private members of a class. A friend class is a special form of friend relationship that allows all member functions of one class to access the private members of another class.

Declaration of Friend Class

To declare a friend class, you need to use the friend keyword in the definition of the class. The syntax is as follows:

class OuterClass {
    // 成员变量和函数

    friend class InnerClass;
};

This will allow all member functions in InnerClass to access the private members of OuterClass, but not other classes.

Practical Case

Let us consider a practical example, where OuterClass represents a class containing sensitive data, and inner class Represents a utility class that needs to access this data.

OuterClass:

class OuterClass {
private:
    int secretData;  // 敏感数据

public:
    // 可以公开访问的数据和方法
};

Inner Class:

class InnerClass {
    friend class OuterClass;  // 声明友元类

public:
    void printData(OuterClass& outerObj) {
        // 访问OuterClass的私有成员 secretData
        cout << "敏感数据：" << outerObj.secretData << endl;
    }
};

In this case, only InnerClass# Member functions in ## can access the secretData member of OuterClass. Other classes cannot access this private member.

Note:

Friend class is a one-way relationship.
• OuterClass can only access the public members of inner class, while inner class can access all members of OuterClass.
Friend classes can be class templates or class instantiations.
• Multiple classes can be designated as friends by using the
• friend class* syntax in the class definition.

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