When to Use C++ Private Inheritance Over Composition

When to use C++ private inheritance over composition?

private inheritance is typically used to represent "implemented-in-terms-of". The main use I have seen is for mixins using private multiple inheritance to build up a child object with the proper functionality from the various mixin parents. This can also be done with composition (which I slightly prefer) but the inheritance method DOES allow you to use using to expose some parent methods publicly, and allows for a slightly more convenient notation when using the mixin methods.

When should I use C++ private inheritance?

Note after answer acceptance: This is NOT a complete answer. Read other answers like here (conceptually) and here (both theoretic and practic) if you are interested in the question. This is just a fancy trick that can be achieved with private inheritance. While it is fancy it is not the answer to the question.

Besides the basic usage of just private inheritance shown in the C++ FAQ (linked in other's comments) you can use a combination of private and virtual inheritance to seal a class (in .NET terminology) or to make a class final (in Java terminology). This is not a common use, but anyway I found it interesting:

class ClassSealer {
private:
   friend class Sealed;
   ClassSealer() {}
};
class Sealed : private virtual ClassSealer
{ 
   // ...
};
class FailsToDerive : public Sealed
{
   // Cannot be instantiated
};

Sealed can be instantiated. It derives from ClassSealer and can call the private constructor directly as it is a friend.

FailsToDerive won't compile as it must call the ClassSealer constructor directly (virtual inheritance requirement), but it cannot as it is private in the Sealed class and in this case FailsToDerive is not a friend of ClassSealer.

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EDIT

It was mentioned in the comments that this could not be made generic at the time using CRTP. The C++11 standard removes that limitation by providing a different syntax to befriend template arguments:

template <typename T>
class Seal {
   friend T;          // not: friend class T!!!
   Seal() {}
};
class Sealed : private virtual Seal<Sealed> // ...

Of course this is all moot, since C++11 provides a final contextual keyword for exactly this purpose:

class Sealed final // ...

Composition over private inheritance

All the time. The real question is, when is private inheritance preferred over composition? Only when virtual functions are required. Else, always favour composition.

Does private inheritance always mean HAS-A?

You example can be implemented through composition like this:

class A {
private:
    class B {
        B(const B&) = delete;
        B& operator=(const B&) = delete;
    } b;
};

A is noncopyable, because its member b is noncopyable.

Private inheritance and composition, which one is best and why?

I prefer to think of inheritance as derived is a kind of base, that basically means public inheritance. In case of private inheritance it more like derived has a base, which IMHO doesn't sound right, because that's IMHO the work for composition not inheritance of any kind. So, since private inheritance and composition essentially mean same thing logically, which to choose? With the example you posted, I'd most certainly go for composition. Why? I tend to think of all kinds of inheritance as a kind of relationship, and with the example you posted, I can't think of a situation where I could say a car is kind of an engine, it simply isn't. It's indeed like a car has an engine, so why would a car inherit from an engine? I see no reason.

Now, indeed there are cases where it's good to have private inheritance, namely boost::noncopyable, with it's ctor/dtor being protected, you'd have hard time instantiating it, and indeed since we want our class to have a noncopyable part, that's the only way to go.

Some style guides (e.g. google c++ style guide) even recommend to never use private inheritance, for reasons similar to what I already written - private inheritance is just a bit confusing.

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

Why does private inheritance increase the probability, as compared to composition, that someone will break my code?

I think the issue isn't that Car can break your Engine code, but rather that by changing Engine, someone can break your Car code. Inheritance represents a much tighter coupling than composition, so that changes in Engine are more likely to break a class that inherits from it rather than contains it. In the case of C++, an even looser coupling is achieved by having Car contain an Engine pointer or smart pointer.

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