Classes

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Friend

A class can have data members that are private. Normally these are not accessible outside the class. However we can declare that a function, class or a class method is a friend and then that function or class methods can access our private members.
File: friend1.cpp

#include <iostream>
using namespace std ;

class point
{
    private:
     int x1 ;
     int y1 ;
     //Does not matter under what access we place the declaration under
     friend void printPoint ( point p1 ) ;
     friend class B ;
};

void printPoint ( point p1 )
{
    cout << p1.x1 << " " << p1.y1 << endl ;
}

class B
{
    void anotherPrintFunction ( point p1 )
    {
        cout << p1.x1 << endl ;
    }
};

int main()
{
    return 0 ;
}

The below example shows how we can declare a member function of another class as a friend. We have 2 classes "base" and "anotherClass" .
File: friend2.cpp

// C++ program to create a member function of another class
// as a friend function
#include <iostream>
using namespace std;

class base; // forward definition needed

// another class in which function is declared
class anotherClass
{
public:
    void memberFunction(base& obj);
};

// base class for which friend is declared
class base {
private:
    int private_variable;

protected:
    int protected_variable;

public:
    base()
    {
        private_variable = 10;
        protected_variable = 99;
    }

    // friend function declaration
    friend void anotherClass::memberFunction(base&);
};




// friend function definition
void anotherClass::memberFunction(base& obj)
{
    cout << "Private Variable: " << obj.private_variable
        << endl;
    cout << "Protected Variable: " << obj.protected_variable;
}


int main()
{
    base baseObject1 ;
    anotherClass object2;
    object2.memberFunction( baseObject1 );

    return 0;
}

Let's say we declare the order of the classes as:

anotherClass
This has a method that relies on the base class:
	void memberFunction(base& obj);

base

Either way we run into a problem. The solution is to use something called a forward declaration:
class base ;
This allows the compiler to compile the code under certain conditions. We are declaring that we are using a class but if the following code has to use the members of the class then the compile will not compile the code. In the above code the "memberFunction" is taking a reference to base and the code compiles. However if instead of defining the method:
void anotherClass::memberFunction(base& obj)
outside the class we instead define it in the class then it creates a problem. We have a forward declaration but the compiler does not know what the data members of the class base are and cannot verify the code in the "memberFunction" .
File: friend3.cpp

// C++ program to create a member function of another class
// as a friend function
#include <iostream>
using namespace std;

class base; // forward definition needed

// another class in which function is declared
class anotherClass
{
public:
    //void memberFunction(base obj);
    void memberFunction(base& obj)
    {
        cout << "Private Variable: " << obj.private_variable
            << endl;
        cout << "Protected Variable: " << obj.protected_variable;
    }

};

// base class for which friend is declared
class base {
private:
    int private_variable;

protected:
    int protected_variable;

public:
    base()
    {
        private_variable = 10;
        protected_variable = 99;
    }

    // friend function declaration
    friend void anotherClass::memberFunction(base&);
};






int main()
{
    base baseObject1 ;
    anotherClass object2;
    object2.memberFunction( baseObject1 );

    return 0;
}

$ g++ friend3.cpp
friend3.cpp: In member function ‘void anotherClass::memberFunction(base&)’:
friend3.cpp:15:49: error: invalid use of incomplete type ‘class base’
   15 |                 cout << "Private Variable: " << obj.private_variable
      |                                                 ^~~
friend3.cpp:6:7: note: forward declaration of ‘class base’
    6 | class base; // forward definition needed

Exercise:

Correct the compiler error in the below code by making class A1 a friend of the Point class.


File: friend_ex1.cpp

#include <iostream>
using namespace std ;

class point
{
    private:
    int x1 ;
    int y1 ;
};

class A1
{
    static void printPoint ( point p1 )
    {
        cout << p1.x1 << " " << p1.y1 << endl ;
    }
};

int main()
{
    return 0 ;
}

Const functions

OO

Smart Pointer