Detailed explanation of C++ friend functions: Application of friend functions in object-oriented design?

C A friend function is a special function that provides access to private and protected members of another class. Non-member functions can interact with a specific class by declaring friend functions. Applications of friend functions include operator overloading, I/O operations and underlying implementation. For example, friend functions can be used to overload operators to support operations between custom data types, as shown in the following case: class Vector { public: friend Vector operator (const Vector& v1, const Vector& v2); };

C Detailed explanation of friend functions: Application in object-oriented design

What is a friend function?

In C, a friend function is a special function that is granted access to private and protected members of another class. This allows non-member functions to interact with a specific class without having to be declared as a member function of the class.

How to declare a friend function?

There are two ways to declare friend functions:

• Declared in the class definition:

class MyClass {
    friend void myFriendFunction();
};

• Use friend Keyword declaration:

class MyClass; // 前向声明

void myFriendFunction() {
    MyClass obj;
    // 访问 MyClass 的私有成员
}

Application of friend function

Friend Functions have many applications in object-oriented design, including:

• Operator overloading: Friend functions can overload operators, such as and * to support operations between custom data types.
• I/O operations: Friend functions can provide I/O functions for classes, such as << and , to simplify object serialization.
• Underlying implementation: Friend functions can access private members of the class to implement complex algorithms or internal data structures.

Practical Case: Operator Overloading

The following code example shows how to use friend functions to overload the operator to support Addition of two Vector objects:

class Vector {
    int x, y;
public:
    Vector(int _x, int _y) : x(_x), y(_y) {}
    friend Vector operator+(const Vector& v1, const Vector& v2);
};

Vector operator+(const Vector& v1, const Vector& v2) {
    return Vector(v1.x + v2.x, v1.y + v2.y);
}

int main() {
    Vector v1(1, 2), v2(3, 4);
    Vector v3 = v1 + v2;  // 使用重载的 + 运算符
    return 0;
}

In the above example, the operator function is a friend function that allows non-member functions to access the Vector Private data members of the class x and y.

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