Difference between private public and protected inheritance

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What is the difference between public  private  and protected inheritance in C        All of the questions I ve found on SO deal with specific cases

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Member in base class   Private   Protected   Public      Inheritance type     nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp Object inherited as   Private                Inaccessible   Private     Private    Protected              Inaccessible   Protected   Protected   Public                 Inaccessible   Protected   Public

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It s essentially the access protection of the public and protected members of the base class in the derived class   With public inheritance  the derived class can see public and protected members of the base   With private inheritance  it can t   With protected  the derived class and any classes derived from that can see them

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I have tried explaining inheritance using a picture below  The main gist is that the private members of parent class are never directly accessible from derived child class but you can use parent class s member function to access the private members of parent class  Private variables are always present in derived class but it cannot be accessed by derived class  Its like its their but you cannot see with your own eyes but if you ask someone form the parent class then he can describe it to you

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Public inheritance models an IS-A relationship  With   class B     class D   public B       every D is a B    Private inheritance models an IS-IMPLEMENTED-USING relationship  or whatever that s called   With   class B     class D   private B       a D is not a B  but every D uses its B in its implementation  Private inheritance can always be eliminated by using containment instead    class B     class D     private       B b        This D  too  can be implemented using B  in this case using its b   Containment is a less tight coupling between types than inheritance  so in general it should be preferred  Sometimes using containment instead of private inheritance is not as convenient as private inheritance  Often that s a lame excuse for being lazy    I don t think anyone knows what protected inheritance models  At least I haven t seen any convincing explanation yet

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I found an easy answer and so thought of posting it for my future reference too   Its from the links http   www learncpp com cpp-tutorial 115-inheritance-and-access-specifiers   class Base   public      int m nPublic     can be accessed by anybody private      int m nPrivate     can only be accessed by Base member functions  but not derived classes  protected      int m nProtected     can be accessed by Base member functions  or derived classes      class Derived  public Base   public      Derived                    Derived s access to Base members is not influenced by the type of inheritance used             so the following is always true           m nPublic   1     allowed  can access public base members from derived class         m nPrivate   2     not allowed  can not access private base members from derived class         m nProtected   3     allowed  can access protected base members from derived class           int main         Base cBase      cBase m nPublic   1     allowed  can access public members from outside class     cBase m nPrivate   2     not allowed  can not access private members from outside class     cBase m nProtected   3     not allowed  can not access protected members from outside class

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These three keywords are also used in a completely different context to specify the visibility inheritance model    This table gathers all of the possible combinations of the component declaration and inheritance model presenting the resulting access to the components when the subclass is completely defined     The table above is interpreted in the following way  take a look at the first row        if a component is declared as public and its class is inherited as public the resulting access is public    An example       class Super       public       int p      private      int q      protected    int r        class Sub   private Super       class Subsub   public Sub       The resulting access for variables p  q  r in class Subsub is none      Another example    class Super       private      int x      protected    int y      public       int z      class Sub   protected Super       The resulting access for variables y  z in class Sub is protected and for variable x is none      A more detailed example    class Super   private      int storage  public      void put int val    storage   val         int  get void       return storage       int main void        Super object       object put 100       object put object get         cout  lt  lt  object get    lt  lt  endl      return 0      Now lets define a subclass   class Sub   Super       int main void        Sub object       object put 100       object put object get         cout  lt  lt  object get    lt  lt  endl      return 0      The defined class named Sub which is a subclass of class named Super or that Sub class is derived from the Super class   The Sub class introduces neither new variables nor new functions  Does it mean that any object of the Sub class inherits all the traits after the Super class being in fact a copy of a Super class    objects   No  It doesn   t   If we compile the following code  we will get nothing but compilation errors saying that put and get methods are inaccessible  Why    When we omit the visibility specifier  the compiler assumes that we are going to apply the so-called private inheritance  It means that all public superclass components turn into private access  private superclass components won t be accessible at all  It consequently means that you are not allowed to use the latter inside the subclass   We have to inform the compiler that we want to preserve the previously used access policy   class Sub   public Super           Don   t be misled  it doesn   t mean that private components of the Super   class  like the storage variable  will turn into public ones in a   somewhat magical way  Private components will remain private  public   will remain public    Objects of the Sub class may do  almost  the same things as their older siblings created from the Super class   Almost  because the fact of being a subclass also means that the class lost access to the private components of the superclass  We cannot write a member function of the Sub class which would be able to directly manipulate the storage variable   This is a very serious restriction  Is there any workaround   Yes   The third access level is called protected  The keyword protected means that the component marked with it behaves like a public one when used by any of the subclasses and looks like a private one to the rest of the world  -- This is true only for the publicly inherited classes  like the Super class in our example  --  class Super   protected      int storage  public      void put int val    storage   val         int  get void       return storage        class Sub   public Super   public      void print void   cout  lt  lt   storage      lt  lt  storage       int main void        Sub object       object put 100       object put object get     1       object print        return 0      As you see in the example code we a new functionality to the Sub class and it does one important thing  it accesses the storage variable from the Super class    It wouldn   t be possible if the variable was declared as private  In the main function scope the variable remains hidden anyway so if you write anything like   object storage   0    The compiler will inform you that it is an error   int Super  storage  is protected   Finally  the last program will produce the following output   storage   101

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To answer that question  I d like to describe member s accessors first in my own words  If you already know this  skip to the heading  next     There are three accessors that I m aware of  public  protected and private    Let   class Base       public          int publicMember      protected          int protectedMember      private          int privateMember        Everything that is aware of Base is also aware that Base contains publicMember  Only the children  and their children  are aware that Base contains protectedMember  No one but Base is aware of privateMember    By  is aware of   I mean  acknowledge the existence of  and thus be able to access    next   The same happens with public  private and protected inheritance  Let s consider a class Base and a class Child that inherits from Base    If the inheritance is public  everything that is aware of Base and Child is also aware that Child inherits from Base  If the inheritance is protected  only Child  and its children  are aware that they inherit from Base  If the inheritance is private  no one other than Child is aware of the inheritance

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Protected data members can be accessed by any classes that inherit from your class  Private data members  however  cannot  Let s say we have the following   class MyClass       private          int myPrivateMember        lol     protected          int myProtectedMember       From within your extension to this class  referencing this myPrivateMember won t work  However  this myProtectedMember will  The value is still encapsulated  so if we have an instantiation of this class called myObj  then myObj myProtectedMember won t work  so it is similar in function to a private data member

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Private   The private members of a base class can only be accessed by members of that base class    Public   The public members of a base class can be accessed by members of that base class  members of its derived class as well as the members which are outside the base class and derived class   Protected   The protected members of a base class can be accessed by members of base class as well as members of its derived class     In short   private   base  protected   base   derived  public   base   derived   any other member

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It has to do with how the public members of the base class are exposed from the derived class       public -  base class s public members will be public  usually the default     protected -  base class s public members will be protected     private -  base class s public members will be private    As litb points out  public inheritance is traditional inheritance that you ll see in most programming languages  That is it models an  IS-A  relationship  Private inheritance  something AFAIK peculiar to C    is an  IMPLEMENTED IN TERMS OF  relationship  That is you want to use the public interface in the derived class  but don t want the user of the derived class to have access to that interface  Many argue that in this case you should aggregate the base class  that is instead of having the base class as a private base  make in a member of derived in order to reuse base class s functionality

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Accessors      Base Class   Derived Class   World                                                                                                                                                  public              y             y           y                                                                                                                                                  protected           y             y           n                                                                                                                                                  private                                           or                y             n           n no accessor                                  y  accessible n  not accessible   Based on this example for java    I think a little table worth a thousand words

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1  Public Inheritance   a  Private members of Base class are not accessible in Derived class   b  Protected members of Base class remain protected in Derived class   c  Public members of Base class remain public in Derived class   So  other classes can use public members of Base class through Derived class object   2  Protected Inheritance   a  Private members of Base class are not accessible in Derived class   b  Protected members of Base class remain protected in Derived class   c  Public members of Base class too become protected members of Derived class   So  other classes can t use public members of Base class through Derived class object  but they are available to subclass of Derived   3  Private Inheritance   a  Private members of Base class are not accessible in Derived class   b  Protected  amp  public members of Base class become private members of Derived class   So  no members of Base class can be accessed by other classes through Derived class object as they are private in Derived class  So  even subclass of Derived  class can t access them

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Limiting the visibility of inheritance will make code not able to see that some class inherits another class  Implicit conversions from the derived to the base won t work  and static cast from the base to the derived won t work either    Only members friends of a class can see private inheritance  and only members friends and derived classes can see protected inheritance    public inheritance   IS-A inheritance  A button is-a window  and anywhere where a window is needed  a button can be passed too    class button   public window         protected inheritance   Protected implemented-in-terms-of  Rarely useful  Used in boost  compressed pair to derive from empty classes and save memory using empty base class optimization  example below doesn t use template to keep being at the point    struct empty pair impl   protected empty class 1    non empty class 2 second      struct pair   private empty pair impl     non empty class 2  amp second         return this- gt second         empty class 1  amp first         return  this     notice we return  this            private inheritance   Implemented-in-terms-of  The usage of the base class is only for implementing the derived class  Useful with traits and if size matters  empty traits that only contain functions will make use of the empty base class optimization   Often containment is the better solution  though  The size for strings is critical  so it s an often seen usage here  template lt typename StorageModel gt  struct string   private StorageModel   public    void realloc            uses inherited function     StorageModel  realloc                public member   Aggregate  class pair   public    First first    Second second      Accessors  class window   public      int getWidth   const        protected member   Providing enhanced access for derived classes  class stack   protected    vector lt element gt  c      class window   protected    void registerClass window descriptor w         private member   Keep implementation details  class window   private    int width          Note that C-style casts purposely allows casting a derived class to a protected or private base class in a defined and safe manner and to cast into the other direction too  This should be avoided at all costs  because it can make code dependent on implementation details - but if necessary  you can make use of this technique

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Summary    Private  no one can see it except for within the class  Protected  Private   derived classes can see it Public  the world can see it   When inheriting  you can  in some languages  change the protection type of a data member in certain direction  e g  from protected to public

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If you inherit publicly from another class  everybody knows you are inheriting and you can be used polymorphically by anyone through a base class pointer   If you inherit protectedly only your children classes will be able to use you polymorphically   If you inherit privately only yourself will be able to execute parent class methods   Which basically symbolizes the knowledge the rest of the classes have about your relationship with your parent class

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class A    public      int x  protected      int y  private      int z      class B   public A          x is public        y is protected        z is not accessible from B     class C   protected A          x is protected        y is protected        z is not accessible from C     class D   private A        private  is default for classes          x is private        y is private        z is not accessible from D      IMPORTANT NOTE  Classes B  C and D all contain the variables x  y and z  It is just question of access   About usage of protected and private inheritance you could read here

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