Private Inheritance

In which I introduce private inheritance, but discourage its use.

When inheriting in C++, you normally see

class Derived : public Base {};

It’s almost as if public is synonymous to inherits from. But did you know there is also private inheritance, and why you (probably) don’t see it a lot?

When inheriting publicly from a base class, all base members will be accessible from the derived class, with the same accessibility as in the base class. Given these classes:

class Base
{
public:
    void pub() {}
private:
    void priv() {}
};

class DerivedPublic : public Base
{
};

class DerivedPrivate : private Base
{
};

Public inheritance results in this:

    DerivedPublic derivedPublic;
    derivedPublic.pub();
    //derivedPublic.priv(); //error: ‘void Base::priv()’ is private

Whereas private inheritance results in this:

    DerivedPrivate derivedPrivate;
    //derivedPrivate.pub(); //error: ‘void Base::pub()’ is inaccessible
    //derivedPrivate.priv(); //error: ‘void Base::priv()’ is private

So why would you want to inherit privately? To allow Derived to access the public members of Base, without exposing them to the users of Derived.

Inside the class, the members are accessible:

class DerivedPrivate2: private Base
{
public:
    void foo() { pub(); }
};

But outside, they are not:

    DerivedPrivate2 derivedPrivate2;
    derivedPrivate2.foo();
    //derivedPrivate2.pub(); //error: ‘void Base::pub()’ is inaccessible

But wait a minute, doesn’t this look a whole lot like the good old inheritance (is-a) vs. composition (has-a)? It does indeed! Private inheritance is really a has-a. And in most circumstances composition and private inheritance are interchangeable. However, since inheritance results in stronger coupling, the general recommendation is to choose composition instead. Here is how DerivedPrivate2 would look using composition:

class NotDerived
{
public:
    void foo() { b.pub(); }
private:
    Base b;
};

Now you may be thinking: “But I read somewhere that you need to use private inheritance if you want to override a virtual Base method?” You probably did, and I’ll get back to that in the next post.

As usual, the code for this blog post is available on GitHub.

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