This is an intermediate conversion course for C++, suitable for second year computing students who may have learned Java or another language in first year.

1. METHOD OVERRIDING
Michael Heron

2. Introduction
• In the last lecture we talked about method overloading.
• In this one we are going to talk about method overriding.
• The two are often confused, but are entirely different systems.
• One is based on the idea of unique identification of
methods.
• The other is based on being able to specialise behaviours
through inheritance.

3. Method Overriding
• When we override a method, we specialise it as the child
class as part of an inherited relationship.
• The usual rules of virtual functions apply here for C++.
• Usually what we do here is subtly alter the functionality
and then pass the results of our specialisation onto the
parent method.

4. Method Overriding
• The chain of invocation for a method in C++ depends on
its virtuality.
• If it is not virtual, the base method gets called always.
• If it is virtual, the most specialised implementation gets called.
• For virtual methods, we then have control over the chain
of invocation.
• We can decide how invocations filter up and down the chain.

5. The Chain of Invocation
• This is a fancy way of saying ‘which functions get
executed when’.
• A well over-ridden function will tend to pass function calls back onto
their parent classes.
• There is nothing to stop you entirely handling a function in
the most specialised instance.
• It’s generally not very good design.
• Violates the principles of encapsulation.

6. Employee (Base Class)
class Employee {
private:
int payscale;
public:
int query_payscale();
void set_payscale (int p);
virtual bool can_hire_and_fire();
virtual bool has_authority (string);
};
bool Employee::has_authority (string action) {
if (action == "work") {
return true;
}
return false;
}

7. Clerical (Derived Class)
class Clerical: public Employee {
public:
bool can_hire_and_fire();
void file_papers();
bool has_authority(string str);
};
bool Clerical::has_authority (string action) {
if (action == "paperwork") {
return true;
}
return Employee::has_authority (action);
}

8. Manager (Derived Class)
class Manager : public Employee {
public:
bool can_hire_and_fire();
bool has_authority (string);
};
bool Manager::has_authority (string action) {
if (action == "managing") {
return true;
}
return Employee::has_authority (action);
}

9. Main Program
int main(int argc, char** argv) {
Employee *cler = new Clerical();
Employee *man = new Manager();
string actions[3] = {"paperwork", "work", "managing"};
for (int i = 0; i < 3; i++) {
cout << "Managers authority for " << actions[i] << " "
<< man->has_authority (actions[i]) << endl;
cout << "Clerical authority for " << actions[i] << " "
<< cler->has_authority (actions[i]) << endl;
}
}

10. Overriding in Java
• Java provides a special keyword, super to refer to a
parent class.
• C++ offers no equivalent due to its support of multiple inheritance.
• Otherwise works identically:
• return super.has_authority (action);
• Overriding simplified by virtue of no virtual functions.

11. Notes on the Chain
• No need for parent classes to define the method.
• Will be passed back up to the most specialised parent that does.
• Scope resolution is used to redirect function calls to
parent classes.
• You can bypass parents if you like, but this is not good practise.

12. Overriding and Encapsulation
• Does overriding break encapsulation?
• Some say it does, and that child classes should not make calls to
parent classes in this way.
• Inheritance is about specialisation.
• Not replacement.
• Properly used, over-riding enhances encapsulation.
• We don’t need to worry about how our methods will be interpreted,
just that they will be.

13. Overloading versus Overriding
• Overriding only works as a mechanism of inheritance.
• You can only override a parent’s implementation.
• You can’t override a method in the class in which it is defined.
• Overloading can work in conjunction with overriding.
• The has_authority method we are using is overridden.
• We could provide a second syntax for that that would be overloaded.
• This has implications for clean object design.

14. Common Overriding Errors
• Mis-spelled function names
• Won’t be picked up when invoked.
• Mismatched parameter lists
• Will overload rather than override.
• Failing to maintain the chain of invocation.
• Make sure you always pass it on.
• Make sure you always pass it on correctly.
• One of the things that super does is resolve to the parent class without you
needing to specify it.
• If you change an inherit chain in C++, you also need to update all
references.

15. Why Override Functions?
• Information hiding ensures we don’t have access to
private data fields.
• In many cases, we simply can’t configure objects without
overriding.
• No need to excessively duplicate functionality.
• We handle what is new, and let the parent class handle the rest.
• Properly distributes authority through an object
inheritance chain.

16. Binding and Polymorphism
• All of this is made possible through the use of inheritance
and polymorphism.
• It’s worth spending a few minutes talking about how this is
actually done.
• C++ makes a distinction between the static type of an
object and its dynamic type.
• Shape *myShape = new Circle();
• myShape has a static type of Shape
• myShape has a dynamic type of Circle

17. Binding and Polymorphism
• The static type is determined at compile time.
• The dynamic type is determined at run-time.
• And it is determined on an ongoing basis.
• This process is known as dynamic binding.
• Dynamic binding has a toll.
• Virtual function lookups at runtime are expensive.
• Virtual functions must be placed and maintained in a virtual function table.
• Static binding is used for functions that are not virtual.
• The function to be executed is decided at compile time and is not changed.

18. Binding Styles
• There is a trade-off between static and dynamic.
• Java makes all methods virtual by default.
• Static binding is more efficient.
• The compiler can do all sorts of tricks to make it work more
efficiently.
• Dynamic binding is more flexible.
• The interface is consistent but the implementation will change.

19. Static Methods in Java
• Static methods in Java are the Java implementation of
static binding.
• Static binding is used for static method calls.
• This is why Java has problems with people calling non-
static methods from static contexts.
• The non-static method is virtual.
• The static method is static.
• You can’t even override a static method in Java.

20. Summary
• Overriding is separate and distinct from overloading.
• Overloading is in addition
• Overriding is instead of
• Maintaining the chain of invocation is important.
• Overriding is predicated on effective use of inheritance
and polymorphism.