What's the point of a final virtual function?

问题:

Wikipedia has the following example on the C++11 final modifier:

struct Base2 {
    virtual void f() final;
};

struct Derived2 : Base2 {
    void f(); // ill-formed because the virtual function Base2::f has been marked final
};

I don't understand the point of introducing a virtual function and immediately marking it as final. Is this simply a bad example, or is there more to it?

回答1:

Typically final will not be used on the base class' definition of a virtual function. final will be used by a derived class that overrides the function in order to prevent further derived types from further overriding the function. Since the overriding function must be virtual normally it would mean that anyone could override that function in a further derived type. final allows one to specify a function which overrides another but which cannot be overridden itself.

For example if you're designing a class hierarchy and need to override a function, but you do not want to allow users of the class hierarchy to do the same, then your might mark the functions as final in your derived classes.

回答2:

It doesn't seem useful at all to me. I think this was just an example to demonstrate the syntax.

One possible use is if you don't want f to really be overrideable, but you still want to generate a vtable, but that is still a horrible way to do things.

回答3:

For a function to be labelled final it must be virtual, i.e., in C++11 §10.3 para. 2:

[...] For convenience we say that any virtual function overrides itself.

and para 4:

If a virtual function f in some class B is marked with the virt-specifier final and in a class D derived from B a function D::f overrides B::f, the program is ill-formed. [...]

i.e., final is required to be used with virtual functions (or with classes to block inheritance) only. Thus, the example requires virtual to be used for it to be valid C++ code.

EDIT: To be totally clear: The "point" asked about concerns why virtual is even used. The bottom-line reason why it is used is (i) because the code would not otherwise compile, and, (ii) why make the example more complicated using more classes when one suffices? Thus exactly one class with a virtual final function is used as an example.

回答4:

I don't understand the point of introducing a virtual function and immediately marking it as final.

The purpose of that example is to illustrate how final works, and it does just that.

A practical purpose might be to see how a vtable influences a class' size.

struct Base2 {
    virtual void f() final;
};
struct Base1 {
};

assert(sizeof(Base2) != sizeof(Base1)); //probably

Base2 can simply be used to test platform specifics, and there's no point in overriding f() since it's there just for testing purposes, so it's marked final. Of course, if you're doing this, there's something wrong in the design. I personally wouldn't create a class with a virtual function just to check the size of the vfptr.

回答5:

Adding to the nice answers above - Here is a well-known application of final (very much inspired from Java). Assume we define a function wait() in a Base class, and we want only one implementation of wait() in all its descendants. In this case, we can declare wait() as final.

For example:

class Base { 
   public: 
       virtual void wait() final { cout << "I m inside Base::wait()" << endl; }
       void wait_non_final() { cout << "I m inside Base::wait_non_final()" << endl; }
}; 

and here is the definition of the derived class:

class Derived : public Base {
      public: 
        // assume programmer had no idea there is a function Base::wait() 

        // error: wait is final
        void wait() { cout << "I am inside Derived::wait() \n"; } 
        // that's ok    
        void wait_non_final() { cout << "I am inside Derived::wait_non_final(); }

} 

It would be useless (and not correct) if wait() was a pure virtual function. In this case: the compiler will ask you to define wait() inside the derived class. If you do so, it will give you an error because wait() is final.

Why should a final function be virtual? (which is also confusing) Because (imo) 1) the concept of final is very close to the concept of virtual functions [virtual functions has many implementations - final functions has only one implementation], 2) it is easy to implement the final effect using vtables.

回答6:

While refactoring legacy code (e.g. removing a method that is virtual from a mother class), this is useful to ensure none of the child classes are using this virtual function.

// Removing foo method is not impacting any child class => this compiles
struct NoImpact { virtual void foo() final {} };
struct OK : NoImpact {};

// Removing foo method is impacting a child class => NOK class does not compile
struct ImpactChildClass { virtual void foo() final {} };
struct NOK : ImpactChildClass { void foo() {} };

int main() {}

回答7:

Here is why you might actually choose to declare a function both virtual and final in a base class:

class A {
    void f();
};

class B : public A {
    void f(); // Compiles fine!
};

class C {
    virtual void f() final;
};

class D : public C {
    void f(); // Generates error.
};

A function marked final has to be also be virtual. Marking a function final prevents you from declaring a function with the same name and signature in a derived class.

回答8:

Instead of this:

public:
    virtual void f();

I find it useful to write this:

public:
    virtual void f() final
        {
        do_f(); // breakpoint here
        }
protected:
    virtual void do_f();

The main reason being that you now have a single place to breakpoint before dispatching into any of potentially many overridden implementations. Sadly (IMHO), saying "final" also requires that you say "virtual."

回答9:

I found another case where for virtual function is useful to be declared as final. This case is part of SonarQube list of warnings. The warning description says:

Calling an overridable member function from a constructor or destructor could result in unexpected behavior when instantiating a subclass which overrides the member function.

For example:
- By contract, the subclass class constructor starts by calling the parent class constructor.
- The parent class constructor calls the parent member function and not the one overridden in the child class, which is confusing for child class' developer.
- It can produce an undefined behavior if the member function is pure virtual in the parent class.

Noncompliant Code Example

class Parent {
  public:
    Parent() {
      method1();
      method2(); // Noncompliant; confusing because Parent::method2() will always been called even if the method is overridden
    }
    virtual ~Parent() {
      method3(); // Noncompliant; undefined behavior (ex: throws a "pure virtual method called" exception)
    }
  protected:
    void         method1() { /*...*/ }
    virtual void method2() { /*...*/ }
    virtual void method3() = 0; // pure virtual
};

class Child : public Parent {
  public:
    Child() { // leads to a call to Parent::method2(), not Child::method2()
    }
    virtual ~Child() {
      method3(); // Noncompliant; Child::method3() will always be called even if a child class overrides method3
    }
  protected:
    void method2() override { /*...*/ }
    void method3() override { /*...*/ }
};

Compliant Solution

class Parent {
  public:
    Parent() {
      method1();
      Parent::method2(); // acceptable but poor design
    }
    virtual ~Parent() {
      // call to pure virtual function removed
    }
  protected:
    void         method1() { /*...*/ }
    virtual void method2() { /*...*/ }
    virtual void method3() = 0;
};

class Child : public Parent {
  public:
    Child() {
    }
    virtual ~Child() {
      method3(); // method3() is now final so this is okay
    }
  protected:
    void method2() override { /*...*/ }
    void method3() final    { /*...*/ } // this virtual function is "final"
};

回答10:

virtual + final are used in one function declaration for making the example short.

Regarding the syntax of virtual and final, the Wikipedia example would be more expressive by introducing struct Base2 : Base1 with Base1 containing virtual void f(); and Base2 containing void f() final; (see below).

Standard

Referring to N3690:

• virtual as function-specifier can be part of decl-specifier-seq
• final can be part of virt-specifier-seq

There is no rule having to use the keyword virtual and the Identifiers with special meaning final together. Sec 8.4, function definitions (heed opt = optional):

function-definition:

attribute-specifier-seq(opt) decl-specifier-seq(opt) declarator virt-specifier-seq(opt) function-body

Practice

With C++11, you can omit the virtual keyword when using final. This compiles on gcc >4.7.1, on clang >3.0 with C++11, on msvc, ... (see compiler explorer).

struct A
{
    virtual void f() {}
};

struct B : A
{
    void f() final {}
};

int main()
{
    auto b = B();
    b.f();
}

PS: The example on cppreference also does not use virtual together with final in the same declaration.

PPS: The same applies for override.