Can a virtual function be overridden by a non-virtual function?

Question

In this code:

class Base {
public:
    virtual void method() = 0;
};

class Derived1 : public Base{
public:
    virtual void method() override { }
};

class Derived2 : public Base{
public:
    void method() override { }
};

Is there any difference between Derived1 and Derived2?

Tranquillize

Derived2::method2 is virtual. – Ba

if syntax allows it. (not checked) then derived2->method() will also be virtual (implicit though) – Carlton

@R.MartinhoFernandes: So the virtual keyword is implied? – Tranquillize

@Alex: Compiles fine for me under gcc-4.7.2 – Tranquillize

Answer 1

From section 10.3 Virtual functions of the c++11 standard (draft n3337) point 2:

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name, parameter-type-list (8.3.5), cv-qualification, and refqualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.

So Derived2::method is also virtual, even though it is not explicitly declared as such.

Unceremonious

Is it ever desirable to declare the member without the virtual keyword? Can GCC be made to spit out a warning when that happens? – Tranquillize

@Eric: Some argue that marking the derived function virtual serves as documentation, others argue that it is unnecessary code bloat. In practice it doesn't really matter either way. – Ferree

With C++11 one can use the override keyword to indicate that a function in a derived class is virtual. – Lithium

@Eric, I agrree with what user1131467. Personally, I prefer it for documentation. As for compiler switches to issue warnings I am unsure. – Unceremonious

Perfect answer. Probably the confusion of the op comes from the fact that in newer languages, such as Java and C#, it is possible to declare a method to stop being virtual. – Mowbray

Answer 2

They are identical.

virtual is optional when actually overriding a function. It is mandatory only when marking a function in the base class.

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