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11.5 — Inheritance and access specifiers

In the previous lessons on inheritance, we’ve been making all of our data members public in order to simplify the examples. In this section, we’ll talk about the role of access specifiers in the inheritance process, as well as cover the different types of inheritance possible in C++.

To this point, you’ve seen the private and public access specifiers, which determine who can access the members of a class. As a quick refresher, public members can be accessed by anybody. Private members can only be accessed by member functions of the same class. Note that this means derived classes can not access private members!

When dealing with inherited classes, things get a bit more complex.

First, there is a third access specifier that we have yet to talk about because it’s only useful in an inheritance context. The protected access specifier restricts access to member functions of the same class, or those of derived classes.

Second, when a derived class inherits from a base class, the access specifiers may change depending on the method of inheritance. There are three different ways for classes to inherit from other classes: public, private, and protected.

To do so, simply specify which type of access you want when choosing the class to inherit from:

If you do not choose an inheritance type, C++ defaults to private inheritance (just like members default to private access if you do not specify otherwise).

That gives us 9 combinations: 3 member access specifiers (public, private, and protected), and 3 inheritance types (public, private, and protected).

The rest of this section will be devoted to explaining the difference between these.

Before we get started, the following should be kept in mind as we step through the examples. There are three ways that members can be accessed:

  • A class can always access it’s own members regardless of access specifier.
  • The public accesses the members of a class based on the access specifiers of that class.
  • A derived class accesses inherited members based on the access specifiers of its immediate parent. A derived class can always access it’s own members regardless of access specifier.

This may be a little confusing at first, but hopefully will become clearer as we step through the examples.

Public inheritance

Public inheritance is by far the most commonly used type of inheritance. In fact, very rarely will you use the other types of inheritance, so your primary focus should be on understanding this section. Fortunately, public inheritance is also the easiest to understand. When you inherit a base class publicly, all members keep their original access specifications. Private members stay private, protected members stay protected, and public members stay public.

This is fairly straightforward. The things worth noting are:

  1. Derived classes can not directly access private members of the base class.
  2. The protected access specifier allows derived classes to directly access members of the base class while not exposing those members to the public.
  3. The derived class uses access specifiers from the base class.
  4. The outside uses access specifiers from the derived class.

To summarize in table form:

Public inheritance
Base access specifier Derived access specifier Derived class access? Public access?
Public Public Yes Yes
Private Private No No
Protected Protected Yes No

Private inheritance

With private inheritance, all members from the base class are inherited as private. This means private members stay private, and protected and public members become private.

Note that this does not affect that way that the derived class accesses members inherited from its parent! It only affects the code trying to access those members through the derived class.

To summarize in table form:

Private inheritance
Base access specifier Derived access specifier Derived class access? Public access?
Public Private Yes No
Private Private No No
Protected Private Yes No

Protected inheritance

Protected inheritance is the last method of inheritance. It is almost never used, except in very particular cases. With protected inheritance, the public and protected members become protected, and private members stay private.

To summarize in table form:

Protected inheritance
Base access specifier Derived access specifier Derived class access? Public access?
Public Protected Yes No
Private Private No No
Protected Protected Yes No

Protected inheritance is similar to private inheritance. However, classes derived from the derived class still have access to the public and protected members directly. The public (stuff outside the class) does not.

Summary

The way that the access specifiers, inheritance types, and derived classes interact causes a lot of confusion. To try and clarify things as much as possible:

First, the base class sets it’s access specifiers. The base class can always access it’s own members. The access specifiers only affect whether outsiders and derived classes can access those members.

Second, derived classes have access to base class members based on the access specifiers of the immediate parent. The way a derived class accesses inherited members is not affected by the inheritance method used!

Finally, derived classes can change the access type of inherited members based on the inheritance method used. This does not affect the derived classes own members, which have their own access specifiers. It only affects whether outsiders and classes derived from the derived class can access those inherited members.

A final example:

Base can access it’s own members without restriction. The public can only access m_nPublic. Derived classes can access m_nPublic and m_nProtected.

D2 can access it’s own members without restriction. D2 can access Base’s members based on Base’s access specifiers. Thus, it can access m_nPublic and m_nProtected, but not m_nPrivate. Because D2 inherited Base privately, m_nPublic, m_nPrivate, and m_nProtected are now private when accessed through D2. This means the public can not access any of these variables when using a D2 object, nor can any classes derived from D2.

D3 can access it’s own members without restriction. D3 can access D2’s members based on D2’s access specifiers. Thus, D3 has access to m_nPublic2 and m_nProtected2, but not m_nPrivate2. D3 access to Base members is controlled by the access specifier of it’s immediate parent. This means D3 does not have access to any of Base’s members because they all became private when D2 inherited them.

11.6 -- Adding, changing, and hiding members in a derived class
Index
11.4 -- Constructors and initialization of derived classes

37 comments to 11.5 — Inheritance and access specifiers

  • qaiser abbas

    You have two classes, Class A and Class B. Being a programmer either you can implement Class B as nested class of Class A or you can inherit Class B from Class A.

    These are two ways to describe the relationship between classes. Discuses the situations in which, concept of public inheritance and nested class is more appropriate to use. Justify with solid reasons.

    please provide the answer

  • Peter Bulyaki

    Very nice and thorough tutorial, however I found one very misleading point:
    For public inheritance, your table says private will become private in the derived class (and you introduce an extra column to say that it is not accessible in the derved class explicitly). This is anything but true, because private will become *inaccessible* in the derived class - not private. Beginners who would take the easy way of learning, would memorize your tables, and they would easily get confused by it.

    What you say is:
    Public -> Public
    Protected -> Protected
    Private -> Private

    This is how I would change your table, putting a "-" to the appropriate cell:
    public -> public
    protected -> protected
    private -> -

    Same applies to protected inheritance:
    public -> protected
    protected -> protected
    private -> -

    And for private inheritance:
    public -> private
    protected -> private
    private -> -

    So in my interpretation, there is an even easier way to memorize all 3*3 cases:
    First, you'd need to point out, that private is private. No matter how you inherit, a private member will NOT be accessible in the derived class.

    So here it is how I'd let people memorize it:
    0. Inheritance specifier is uninterpretable for private members.

    1. PUBLIC inheritance: Everything remains the SAME in the derived class:
    public -> public
    protected -> protected

    2. PROTECTED inheritance: Everything becomes PROTECTED in the derived class:
    public -> protected
    protected -> protected

    3. PRIVATE inheritance: Everything becomes PRIVATE in the derived class:
    public -> private
    protected -> private

    This is how I was taught it, and I think that it is much simpler this way.

    • Nikhil Singhal

      Thanks Peter

      your comment makes Alex's article more understandable, specially for Beginners, most of the times they become confused at this point, but if they follow your table they never confused LIKE ME :)

      Thanks & Regards
      Nikhil Singhal

    • singhchauhan.nitin

      Peter you made it more clear to understand the hierarchy of inheritance .. thanks a lot..

  • ROHITH

    ITS SUPERB AND THANKS A LOT. IT GIVES A OVER ALL VIEW OF THE ACCESS SPECIFIERS STORY.

    THANKS.

  • Kevin

    Excellent tutorial.
    The best I've seen so far.
    Helped me a lot.
    Thank you!

  • shruthi

    Thanks!. Good explanation of the topic.

  • Rollie

    Great explanation - thanks!

  • auasp

    Is the first table on Public inheritance correct?
    I think all the entries in "derived access specifier" column should be Public.

  • ozzy

    Very nice explanation !

  • Renuka S
    Found it great! Thanks!!
  • Vikas

    Excellent.. amazing simplicity in way of teaching... kudos to you...

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