C++ override specifier without virtual? Does override imply virtual?

Question

Linked question is not the same - and does not even mention override

Edit: The new list of duplicates contains one legitimate duplicate, which I did not find from search.

I was not aware prior to asking this that the choice of whether or not to use virtual in derived class members was going to be a contentious issue for some.

---

I have just encountered some source code which looks like this:

class A
{
    virtual void method();
};

class B : public A
{
    void method() override;
}

I am unsure of how to interpret this, even after reading this.

Does override imply virtual here? void B::method() is not marked as a virtual function, but it is marked as override. So why does this work, and not result in a compilation error?

Is there any difference between the following? (In class B)

• void method() override;
• virtual void method() override;

Answer 1

The answer you're looking for is in https://en.cppreference.com/w/cpp/language/virtual

If some member function vf is declared as virtual in a class Base, and some class Derived, which is derived, directly or indirectly, from Base, has a declaration for member function with the same name parameter type list (but not the return type) cv-qualifiers ref-qualifiers Then this function in the class Derived is also virtual (whether or not the keyword virtual is used in its declaration) and overrides Base::vf (whether or not the word override is used in its declaration).

Answer 2

Does override imply virtual here?

Yes. Only virtual methods can be overridden. override on a non-virtual method is an error.

void B::method() is not marked as a virtual function,[..]

It is sufficient to declare a method to be virtual in the base class. In all classes deriving from A the method is virtual. Adding the virtual specifier to B::method would be redundant.

Is there any difference between the following? [...]

It is good for readability to mention also in the derived class that a method is virtual. Though, as only virtual methods can be overridden, it is sufficient to specify override to make it clear that it is a virtual method.

Answer 3

Yes, override implies virtual in the sense that if the function was not already declared virtual in a base class then override would be an error (and diagnosed as such by the compiler) and adding both virtual and override to the same function is redundant (just as adding the virtual keyword in derived classes is redundant - functions that are virtual in base classes are always virtual in derived classes, even if not explicitly marked as such). However, that also means that override does not make a function virtual if it wasn't already.

The code you show is perfectly sensible. It clearly communicated (via the override keyword) that the function is virtual and also overrides a virtual function in a base class (all with just one keyword).

Answer 4

The override keyword simply means that this function is overriding a virtual function from the base class.

Best practice is to use virtual in base classes and override in derived (mainly for readability and convenience).

Another important keyword is the "final" keyword (comes right after override at the function header)

You can use it to specify this is the last override on the virtual function, thus it disables any further overriding (this, of course, is only useful when dealing with a derived class of an derived class).