When do you need to use friend functions?

Scenarios for using friend functions in C include: 1. Operator overloading; 2. Cross-class access; 3. Global functions; 4. Testing. Friend functions can access private members of other classes, but they reduce encapsulation, so use them sparingly only when necessary and make sure to provide only the necessary access.

When to use friend functions

In C, a friend function is a special function that can access a certain Private and protected members of a class. In certain situations, using friend functions can provide convenience and flexibility.

Use occasions

• Operator overloading: When an operator needs to be overloaded to operate on an object, such as for a class implementation or - operator.
• Cross-class access: When you need to access a private or protected member from another class.
• Global function: When you need to create a global function that can access private members of other classes.
• Testing: When you need to access private members from a test file for unit testing.

Syntax

The syntax for declaring a friend function is as follows:

friend 返回值类型 函数名(参数列表);

The syntax for declaring a class as a friend is as follows:

class 类名 {
    // ...
    friend 返回值类型 函数名(参数列表);
    // ...
};

Practical case

Consider a Date class representing a date, which has private members day, month and year. Now, we want to implement a isLeapYear function to check whether the specified year is a leap year.

class Date {
private:
    int day, month, year;

public:
    // ...

    friend bool isLeapYear(int year);
};

bool isLeapYear(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

In this case, the friend function isLeapYear can access the private member year of the Date class in order to calculate whether it is a leap year .

Other notes

• Using friend functions will weaken the encapsulation of the class, so use it with caution.
• Use friend functions only when absolutely necessary, giving priority to accessor and modifier methods.
• Ensure that friend functions have only the necessary permissions to achieve their goals and avoid providing excessive access to private members.

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