Difference Between Public, Private, and Protected Inheritance in C++

What is the difference between public, private, and protected inheritance in C++?

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.

C++ why use public, private or protected inheritance?

Use public inheritance to reflect an is-a relationship. This is the main use for inheritance, especially in combination with virtual functions. It allows re-use of interface, not just of old code by new code, but also re-use of new code by old code! (because of virtual function dispatch at runtime).

In exceptional circumstances, use private inheritance to reflect an is-implemented-in-terms-of relationship. This is a commonly overused pattern, often the equivalent goal can be reached through composition (having the would-be base class as a data member). Another drawback is that you can easily have multiple inheritance of the same base class (twice or more removed) leading to the so-called Diamond Problem.

Avoid using protected inheritance, it suggest that your class interface is client-dependent (derived classes versus the world). Often this is due to classes having multiple responsiblities, suggesting a refactoring into separate classes is appropriate.

What is the difference between private and protected members of C++ classes?

Private members are only accessible within the class defining them.

Protected members are accessible in the class that defines them and in classes that inherit from that class.

Edit: Both are also accessible by friends of their class, and in the case of protected members, by friends of their derived classes.

Edit 2: Use whatever makes sense in the context of your problem. You should try to make members private whenever you can to reduce coupling and protect the implementation of the base class, but if that's not possible then use protected members. Check C++ FAQ for a better understanding of the issue. This question about protected variables might also help.

Why do we actually need Private or Protected inheritance in C++?

It is useful when you want to have access to some members of the base class, but without exposing them in your class interface. Private inheritance can also be seen as some kind of composition: the C++ faq-lite gives the following example to illustrate this statement

class Engine {
 public:
   Engine(int numCylinders);
   void start();                 // Starts this Engine
};

class Car {
  public:
    Car() : e_(8) { }             // Initializes this Car with 8 cylinders
    void start() { e_.start(); }  // Start this Car by starting its Engine
  private:
    Engine e_;                    // Car has-a Engine
};

To obtain the same semantic, you could also write the car Class as follow:

class Car : private Engine {    // Car has-a Engine
 public:
   Car() : Engine(8) { }         // Initializes this Car with 8 cylinders
   using Engine::start;          // Start this Car by starting its Engine
}; 

However, this way of doing has several disadvantages:

• your intent is much less clear
• it can lead to abusive multiple inheritance
• it breaks the encapsulation of the Engine class since you can access its protected members
• you're allowed to override Engine virtual methods, which is something you don't want if your aim is a simple composition

the protected and private member variables in C++ inheritance

All the member of the base class are part of the derived class. However, the derived class can only access members that are public or protected.

Declaring a member of the same name as a member of a Base class "shadows" the member of the Base class. That is the Derived class has its own independent variable that happens to have the same name as the base class version.

This is a personal choice, but I find using variables to communicate between base classes and derived classes leads to messier code so I tend to either make member variables private or use the PIMPL pattern.

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