How to declare and use friend functions in C++?

A friend function in C is a special function that can access private/protected members of other classes. The friend keyword needs to be used when declaring a friend function, such as: declaring a friend function: friend void printValue(const MyClass& obj); using a friend function: a friend function can be used like a normal function and can access private/protected members; Practical case: In a logging system or testing framework, friend functions can access class private data and implement specific functions; Note: Friend functions should be used with caution, complete signatures must be specified, and protected members cannot be directly accessed unless the member Also declared as friend.

Declaring and using friend functions in C

A friend function is a special global function or method that can Access private and protected members of other classes. This is useful in situations where you need to access its internal data or operations from outside the class.

Declaring Friend Functions

To declare a friend function, use the friend keyword as follows:

class MyClass {
private:
    int value;

public:
    friend void printValue(const MyClass& obj);
};

// 友元函数声明
void printValue(const MyClass& obj);

The above statement makes the printValue function a friend function of MyClass.

Using Friend Functions

Once a friend function is declared, it can be used like any other global function. Friend functions have privileged access to private and protected members of the class. For example, in the following code, the printValue function prints the private member value of MyClass:

#include <iostream>

using namespace std;

class MyClass {
private:
    int value;

public:
    friend void printValue(const MyClass& obj);
};

void printValue(const MyClass& obj) {
    cout << "Value: " << obj.value << endl;
}

int main() {
    MyClass obj;
    obj.value = 10;
    printValue(obj);

    return 0;
}

Output:

Value: 10

Practical Case

The following is a real-life case using friend functions:

• Logging system:Create a Friend functions to record class operations or events. This allows the logging system to access a class's private data without modifying the class itself.
• Testing framework: Create friend functions to access private members of the class to facilitate unit testing. This allows the testing framework to inspect and verify the internal state of the class.

Note

• Friend functions should be used with caution because they can break the encapsulation of a class.
• When declaring friend functions, be sure to specify their complete signatures, including parameter types and return types.
• Friend functions cannot directly access protected members of a class unless the member is also declared friend.

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