Traditional C++ inheritance mostly can’t be controlled

In C++98 any class can be inherited. (An extremely obscure pattern will prevent this, but it has down sides.) Sometimes this makes sense: it’s natural to subclass an abstract List class as LinkedList, or LinkedList as CircularLinkedList. But sometimes this doesn’t make sense. StringBuilder certainly could inherit from Vector<char>, but doing so might expose many Vector methods that don’t belong in the StringBuilder concept. It would be more sensible for StringBuilder to contain a private Vector<char> which StringBuilder member methods manipulated. Preventing Vector from being used as a base class would be one way (not necessarily the best one) to avoid this conceptual error. But C++98 doesn’t let you easily do that.

Even when inheritance is desired, sometimes you don’t want completely-virtual methods. Sometimes you’d like a class to implement a virtual method (virtual perhaps because its base declared it so) which derived classes can’t override. Perhaps you want to rely on that method being implemented only by your base class, or perhaps you want it “fixed” as an optimization. Again in C++98, you can’t do this: public virtual functions are overridable.

C++11′s contextual final keyword

C++11 introduces a contextual final keyword for these purposes. To prevent a class from being inheritable, add final to its definition just after the class name (the class can’t be unnamed).

struct Base1 final
{
  virtual void foo();
};

// ERROR: can't inherit from B.
struct Derived1 : public Base1 { };

struct Base2 { };

// Derived classes can be final too.
struct Derived2 final : public Base2 { };

Similarly, a virtual member function can be marked as not overridable by placing the contextual final keyword at the end of its declaration, before a terminating semicolon, body, or = 0.

struct Base
{
  virtual void foo() final;
};

struct Derived : public Base
{
  // ERROR: Base::foo was final.
  virtual void foo() { }
};

Introducing MOZ_FINAL

mfbt now includes support for marking classes and virtual member functions as final using the MOZ_FINAL macro in mozilla/Attributes.h. Simply place it in the same position as final would occur in the C++11 syntax:

#include "mozilla/Attributes.h"

class Base
{
  public:
    virtual void foo();
};

class Derived final : public Base
{
  public:
    /*
     * MOZ_FINAL and MOZ_OVERRIDE are composable; as a matter of
     * style, they should appear in the order MOZ_FINAL MOZ_OVERRIDE,
     * not the other way around.
     */
    virtual void foo() MOZ_FINAL MOZ_OVERRIDE { }
    virtual void bar() MOZ_FINAL;
    virtual void baz() MOZ_FINAL = 0;
};

MOZ_FINAL expands to the C++11 syntax or its compiler-specific equivalent whenever possible, turning violations of final semantics into compile-time errors. The same compilers that usefully expand MOZ_OVERRIDE also usefully expand MOZ_FINAL, so misuse will be quickly noted.

One interesting use for MOZ_FINAL is to tell the compiler that one worrisome C++ trick sometimes isn’t. This is the virtual-methods-without-virtual-destructor trick. It’s used when a class must have virtual functions, but code doesn’t want to pay the price of destruction having virtual-call overhead.

class Base
{
  public:
    virtual void method() { }
    ~Base() { }
};

void destroy(Base* b)
{
  delete b; // may cause a warning
}

Some compilers warn when they instantiate a class with virtual methods but without a virtual destructor. Other compilers only emit this warning when a pointer to a class instance is deleted. The reason is that in C++, behavior is undefined if the static type of the instance being deleted isn’t the same as its runtime type and its destructor isn’t virtual. In other words, if ~Base() is non-virtual, destroy(new Base) is perfectly fine, but destroy(new DerivedFromBase) is not. The warning makes sense if destruction might miss a base class — but if the class is marked final, it never will! Clang silences its warning if the class is final, and I hope that MSVC will shortly do the same.

What about NS_FINAL_CLASS?

As with MOZ_OVERRIDE we had a gunky XPCOM static-analysis NS_FINAL_CLASS macro for final classes. (We had no equivalent for final methods.) NS_FINAL_CLASS too was misplaced as far as C++11 syntax was concerned, and it too has been deprecated. Almost all uses of NS_FINAL_CLASS have now been removed (the one remaining use I’ve left for the moment due to an apparent Clang bug I haven’t tracked down yet), and it shouldn’t be used.

(Side note: In replacing NS_FINAL_CLASS with MOZ_FINAL, I discovered that some of the existing annotations have been buggy for months! Clearly no one’s done static analysis builds in awhile. The moral of the story: compiler static analyses that happen for every single build are vastly superior to user static analyses that happen only in special builds.)

Summary

If you don’t want a class to be inheritable, add MOZ_FINAL to its definition after the class name. If you don’t want a virtual member function to be overridden in derived classes, add MOZ_FINAL at the end of its declaration. Some compilers will then enforce your wishes, and you can rely on these requirements rather than hope for the best.