What are the declaration rules for C++ friend functions?

The rules for declaration of friend functions are as follows: the declaration must be outside the class. Can be declared as a member function or global function. Objects of a class can be passed through pointers or references.

C Declaration rules for friend functions

Friend function declaration

A friend function is a special function that can access private members of a class. To declare a friend function in C, use the friend keyword as follows:

// 声明 MyClass 类的友元函数 printInfo()
friend void printInfo(const MyClass& obj);

Declaration Rules

The following are in C Rules for declaring friend functions:

• The declaration of a friend function must be outside the class.
• Friend functions can be declared as member functions (with scope resolver) or global functions (without scope resolver).
• Objects of a class can be passed through pointers or references.

Practical case

Consider the following MyClass class, which has private member variables _data:

class MyClass {
private:
    int _data;
public:
    // ...
};

We can define a friend function printInfo() to access _data:

// 声明 printInfo() 为 MyClass 的友元函数
friend void printInfo(const MyClass& obj) {
    std::cout << "Data: " << obj._data << std::endl;
}

In the main function, we can instantiate MyClass Object and call friend function to print private data:

int main() {
    MyClass obj;
    obj._data = 42;  // 访问私有成员（仅在友元函数中允许）
    printInfo(obj);  // 调用友元函数
    return 0;
}

Output:

Data: 42

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