To Override or Not to Override: Is Explicitly Declaring 'virtual' Necessary When Overriding Virtual Functions in C ?

Overriding Virtual Functions in Derived C Classes: Necessity or Not?

C provides the "virtual" keyword to enable polymorphic behavior in derived classes. Polymorphism allows different classes to implement different versions of the same function. However, there is a question that arises: is it necessary to specify "virtual" when overriding virtual functions in derived classes?

Consider the following struct definition:

struct A {
    virtual void hello() = 0;
};

This defines an abstract base class A with a pure virtual function hello(). Now, let's examine two approaches to overriding this function in a derived class B:

Approach #1:
struct B : public A {
    virtual void hello() { ... }
};

Approach #2:
struct B : public A {
    void hello() { ... }
};

The question is: is there any difference between these two approaches?

Answer:

The answer is no, there is no difference in behavior between these two approaches. However, there is a subtle distinction in their usage.

• Approach #1: Explicitly specifies the overridden function as "virtual." This is necessary if you want to override a virtual function in a derived class that may have been declared as virtual in one of its base classes. In this case, using "virtual" ensures that the function override is done polymorphically.
• Approach #2: Does not explicitly specify the overridden function as "virtual." This is valid when the overridden function is already declared as virtual in the base class. The "virtual" keyword is implicitly inherited in this case, making the override polymorphic.

In the example provided, the overridden function is already declared as virtual in the base class A. Therefore, both approaches will result in polymorphic behavior. The choice between the two approaches depends on the clarity and consistency you prefer in your code.

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