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copied!<h2>what are Access Specifiers?</h2> <p>There are 3 <code>access specifiers</code> for a class/struct/Union in C++. These access specifiers define how the members of the class can be accessed. Of course, any member of a class is accessible within that class(Inside any member function of that same class). Moving ahead to type of access specifiers, they are: </p> <p><strong>Public</strong> - The members declared as Public are accessible from outside the Class through an object of the class. </p> <p><strong>Protected</strong> - The members declared as Protected are accessible from outside the class <strong>BUT</strong> only in a class derived from it. </p> <p><strong>Private</strong> - These members are only accessible from within the class. No outside Access is allowed.</p> <p>An Source Code Example:</p> <pre><code>class MyClass { public: int a; protected: int b; private: int c; }; int main() { MyClass obj; obj.a = 10; //Allowed obj.b = 20; //Not Allowed, gives compiler error obj.c = 30; //Not Allowed, gives compiler error } </code></pre> <hr> <h2>Inheritance and Access Specifiers</h2> <p>Inheritance in C++ can be one of the following types: </p> <ul> <li><strong><code>Private</code> Inheritance</strong> </li> <li><strong><code>Public</code> Inheritance</strong> </li> <li><strong><code>Protected</code> inheritance</strong> </li> </ul> <p>Here are the member access rules with respect to each of these: </p> <blockquote> <p><strong><em>First and most important rule <code>Private</code> members of a class are never accessible from anywhere except the members of the same class.</em></strong> </p> </blockquote> <h2>Public Inheritance:</h2> <blockquote> <p>All <code>Public</code> members of the Base Class become <code>Public</code> Members of the derived class &<br> All <code>Protected</code> members of the Base Class become <code>Protected</code> Members of the Derived Class. </p> </blockquote> <p>i.e. No change in the Access of the members. The access rules we discussed before are further then applied to these members. </p> <p>Code Example: </p> <pre><code>Class Base { public: int a; protected: int b; private: int c; }; class Derived:public Base { void doSomething() { a = 10; //Allowed b = 20; //Allowed c = 30; //Not Allowed, Compiler Error } }; int main() { Derived obj; obj.a = 10; //Allowed obj.b = 20; //Not Allowed, Compiler Error obj.c = 30; //Not Allowed, Compiler Error } </code></pre> <h2>Private Inheritance:</h2> <blockquote> <p>All <code>Public</code> members of the Base Class become <code>Private</code> Members of the Derived class &<br> All <code>Protected</code> members of the Base Class become <code>Private</code> Members of the Derived Class.</p> </blockquote> <p>An code Example: </p> <pre><code>Class Base { public: int a; protected: int b; private: int c; }; class Derived:private Base //Not mentioning private is OK because for classes it defaults to private { void doSomething() { a = 10; //Allowed b = 20; //Allowed c = 30; //Not Allowed, Compiler Error } }; class Derived2:public Derived { void doSomethingMore() { a = 10; //Not Allowed, Compiler Error, a is private member of Derived now b = 20; //Not Allowed, Compiler Error, b is private member of Derived now c = 30; //Not Allowed, Compiler Error } }; int main() { Derived obj; obj.a = 10; //Not Allowed, Compiler Error obj.b = 20; //Not Allowed, Compiler Error obj.c = 30; //Not Allowed, Compiler Error } </code></pre> <h2>Protected Inheritance:</h2> <blockquote> <p>All <code>Public</code> members of the Base Class become <code>Protected</code> Members of the derived class &<br> All <code>Protected</code> members of the Base Class become <code>Protected</code> Members of the Derived Class. </p> </blockquote> <p>A Code Example:</p> <pre><code>Class Base { public: int a; protected: int b; private: int c; }; class Derived:protected Base { void doSomething() { a = 10; //Allowed b = 20; //Allowed c = 30; //Not Allowed, Compiler Error } }; class Derived2:public Derived { void doSomethingMore() { a = 10; //Allowed, a is protected member inside Derived & Derived2 is public derivation from Derived, a is now protected member of Derived2 b = 20; //Allowed, b is protected member inside Derived & Derived2 is public derivation from Derived, b is now protected member of Derived2 c = 30; //Not Allowed, Compiler Error } }; int main() { Derived obj; obj.a = 10; //Not Allowed, Compiler Error obj.b = 20; //Not Allowed, Compiler Error obj.c = 30; //Not Allowed, Compiler Error } </code></pre> <p>Remember the same access rules apply to the classes and members down the inheritance hierarchy. </p> <hr> <h2>Important points to note:</h2> <p><strong><em>- Access Specification is per-Class not per-Object</em></strong></p> <p>Note that the access specification C++ work on per-Class basis and not per-object basis.<br> A good example of this is that in a copy constructor or Copy Assignment operator function, all the members of the object being passed can be accessed.</p> <p><strong><em>- A Derived class can only access members of its own Base class</em></strong></p> <p>Consider the <strong><a href="http://ideone.com/5iAeM" rel="noreferrer">following code example</a></strong>:</p> <pre><code>class Myclass { protected: int x; }; class derived : public Myclass { public: void f( Myclass& obj ) { obj.x = 5; } }; int main() { return 0; } </code></pre> <p>It gives an compilation error: </p> <blockquote> <p>prog.cpp:4: error: ‘int Myclass::x’ is protected</p> </blockquote> <p>Because the derived class can only access members of its <strong><em>own Base Class</em></strong>. Note that the object <code>obj</code> being passed here is no way related to the <code>derived</code> class function in which it is being accessed, it is an altogether different object and hence <code>derived</code> member function cannot access its members.</p> <hr> <h2>What is a <code>friend</code>? How does <code>friend</code> affect access specification rules?</h2> <p>You can declare a function or class as <code>friend</code> of another class. When you do so the access specification rules do not apply to the <code>friend</code>ed class/function. The class or function can access all the members of that particular class. </p> <blockquote> <p><strong><em>So do <code>friend</code>s break Encapsulation?</em></strong></p> </blockquote> <p>No they don't, On the contrary they enhance Encapsulation!</p> <p><code>friend</code>ship is used to indicate a <strong><em>intentional strong coupling</em></strong> between two entities.<br> If there exists a special relationship between two entities such that one needs access to others <code>private</code> or <code>protected</code> members but You do not want <strong><em>everyone</em></strong> to have access by using the <code>public</code> access specifier then you should use <code>friend</code>ship. </p>

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