Do Derived Classes Inherit Default Parameters in Virtual Functions?

Virtual Functions with Default Parameters

Virtual functions are commonly used in object-oriented programming for polymorphism. They can have default parameter values, and the question arises whether derived classes inherit these defaults or specify their own.

Do Derived Classes Inherit Default Values?

According to the C Standard, derived classes do not inherit the default values of virtual functions from the base class.

Which Default is Used?

The default value used in a virtual function call depends on the static type of the object being called. If you call a virtual function through a base class object or reference, the default value from the base class will be used. However, if you call through a derived class object or reference, the default value from the derived class will be used.

Compiler Behavior and Recommended Practices

Different compilers may handle virtual function defaults differently. However, the C Standard dictates that the default values are not inherited and should be specified explicitly in derived classes.

Example

Consider the following example:

struct Base {
  virtual void f(int a = 7);
};
struct Derived : public Base {
  void f(int a);
};

In this example, the f function in the Derived class does not inherit the default value of 7 from the base class. When calling f through a Derived class object, the call should specify the argument explicitly, as shown below:

Derived d;
d.f(10); // Call `Derived::f(int a)` with a = 10

Conclusion

While virtual functions can have default parameters, these defaults are not inherited by derived classes. The default value used depends on the static type of the object being called. Following the C Standard recommendations and explicitly specifying default values in derived classes is considered best practice.

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