Virtual functions should specify exactly one of `virtual`, `override`, or `final`

Reason

Readability. Detection of mistakes. Writing explicit virtual, override, or final is self-documenting and enables the compiler to catch mismatch of types and/or names between base and derived classes. However, writing more than one of these three is both redundant and a potential source of errors.

It's simple and clear:

• virtual means exactly and only "this is a new virtual function."
• override means exactly and only "this is a non-final overrider."
• final means exactly and only "this is a final overrider."

Example, bad

struct B {
    void f1(int);
    virtual void f2(int) const;
    virtual void f3(int);
    // ...
};

struct D : B {
    void f1(int);        // bad (hope for a warning): D::f1() hides B::f1()
    void f2(int) const;  // bad (but conventional and valid): no explicit override
    void f3(double);     // bad (hope for a warning): D::f3() hides B::f3()
    // ...
};

Example, good

struct Better : B {
    void f1(int) override;        // error (caught): Better::f1() hides B::f1()
    void f2(int) const override;
    void f3(double) override;     // error (caught): Better::f3() hides B::f3()
    // ...
};

Discussion

We want to eliminate two particular classes of errors:

• implicit virtual: the programmer intended the function to be implicitly virtual and it is (but readers of the code can't tell); or the programmer intended the function to be implicitly virtual but it isn't (e.g., because of a subtle parameter list mismatch); or the programmer did not intend the function to be virtual but it is (because it happens to have the same signature as a virtual in the base class)
• implicit override: the programmer intended the function to be implicitly an overrider and it is (but readers of the code can't tell); or the programmer intended the function to be implicitly an overrider but it isn't (e.g., because of a subtle parameter list mismatch); or the programmer did not intend the function to be an overrider but it is (because it happens to have the same signature as a virtual in the base class -- note this problem arises whether or not the function is explicitly declared virtual, because the programmer might have intended to create either a new virtual function or a new non-virtual function)

Note: On a class defined as final, each individual virtual function should use either override or final; there is no semantic difference in this case.

Note: Use final on functions sparingly. It does not necessarily lead to optimization, and it precludes further overriding.

Enforcement

• Compare virtual function names in base and derived classes and flag uses of the same name that do not override.
• Flag overrides with neither override nor final.
• Flag function declarations that use more than one of virtual, override, and final.