friend.md

The friend keyword in C++ is used to grant non-member functions or other classes access to private (and protected) members of a class. It's a powerful feature, but with power comes responsibility; it can break encapsulation if misused.

Friend Function

A friend function is not a member of the class, but it can access the private and protected members of the class.

class Box 
{
private:
    int length;

public:
    Box(int l): length(l) {}
    friend int getLength(const Box& b);
};

int getLength(const Box& b) {
    return b.length;  // Can access private member of Box
}

Characteristics:

• A friend function is not a member of the class, so it cannot be called using the object of the class.

• It's defined outside the class's scope but needs to be declared inside the class with the friend keyword.

• It cannot access the class's private members directly. Instead, they should be passed as arguments.

Friend Class

When one class is declared as a friend of another class, the friend class can access the private and protected members of the other class.

class Box 
{
private:
    int length;

public:
    Box(int l): length(l) {}
    friend class BoxInspector;  // Declaring BoxInspector as a friend class
};

class BoxInspector 
{
public:
    int inspectLength(const Box& b) {
        return b.length;  // Can access private member of Box
    }
};

Characteristics:

• Just because BoxInspector is a friend of Box doesn't mean Box has access to private members of BoxInspector. The friend relationship isn't mutual unless explicitly defined as such.

• Being a friend class doesn't grant any special relationship in terms of inheritance. It's strictly about access.

Friend Member Function

A class can make certain member functions of another class its friend, rather than the whole class.

class Box 
{
private:
    int length;

public:
    Box(int l): length(l) {}
    
    // Only the inspectLength function of BoxInspector is a friend
    friend int BoxInspector::inspectLength(const Box& b);
};

class BoxInspector 
{
public:
    int inspectLength(const Box& b) {
        return b.length;  // Can access private member of Box
    }
};

Characteristics:

• It provides a fine-grained control by allowing only specific member functions from another class to have access.