This is an introductory lecture on C++, suitable for first year computing students or those doing a conversion masters degree at postgraduate level.

1. Inheritance
Michael Heron

2. Introduction
• Last week we discussed object orientation a little bit.
• Introducing one of the key principles of object orientation –
encapsulation.
• This week we will round off our discussion of object
orientation.
• Today we talk about the principle of inheritance.
• Tomorrow we’re going to discuss general object design.

3. Encapsulation Revisited
• One of the things that makes encapsulation a powerful tool is
that we have one distinct unit to represent a collection of data
and methods.
• We can leverage this representation in various ways.
• It is the foundation of several other object oriented principles.
• One of the things that extend from this system is the principle
of inheritance.

4. Inheritance
• The concept of inheritance is borrowed from the natural
world.
• You get traits from your parents and you pass them on to your
offspring.
• In C++, any class can be the parent of any other object.
• The child class gains all of the methods and attributes of the
parent.
• This can be modified, more on that later.

5. Inheritance in the Natural
World

6. Inheritance
• Inheritance is a powerful concept for several reasons.
• Ensures consistency of code across multiple different objects.
• Allows for code to be collated in one location with the
concomitant impact on maintainability.
• Changes in the code rattle through all objects making use of it.
• Supports for code re-use.
• Theoretically…

7. Inheritance
• In C++, the most general case of a code system belongs at the
top of the inheritance hierarchy.
• It is successively specialised into new and more precise
implementations.
• Children specialise their parents
• Parents generalise their children.
• Functionality and attributes belong to the most general class
in which they are cohesive.

8. Inheritance in C++
• At its basic level, relatively simple.
• A single colon is used in place of the extends keyword.
• We also include public before the class name.
• We have no access to variables and methods defined as
private.
• This is where the visibility modifiers we discussed last week come
into play.

9. Inheritance in C++
class Car {
private:
float price;
public:
void set_price (float p);
float query_price();
};
This is the header file for our class – remember classes in C++ are spread over two
files. The header file defines the class structure, and includes none of the code.

10. Inheritance in C++
#include "car.h”
void Car::set_price (float p) {
price = p;
}
float Car::query_price() {
return price;
}
This is the cpp file for the class – it defines the code, but doesn’t define what the class
structure is. That’s left for the header file.

11. Inheritance in C++
class ExtendedCar : public Car {
private:
float mpg;
public:
void set_mpg (float f);
float query_mpg();
};
float ExtendedCar::query_mpg() {
return mpg;
}
void ExtendedCar::set_mpg (float f) {
mpg = f;
}

12. Constructors
• It is often useful for us to be able to setup an object with
some starting values to its attributes.
• We can do this using a constructor method.
• This is a special method called whenever an object is created.
• Constructor methods work the same way as other methods.
• They just have a slightly different format.

13. Constructors in Theory
• It is called automatically when an object is initialized.
• When we create an object from a class.
• Can be used for any purpose, but most often used for
initializing data fields.
• We can provide a range of constructor methods inside an
object to deal with all eventualities.

14. Constructors
• Constructors must have the same name as the class in which
they are defined.
• Constructors have no return type.
• They are not void, the return type is actually omitted.
• They are usually declared as public.
• A well designed object will often provide several constructors.
• This is perfectly valid, provided they all have unique method
signatures.

15. Constructors in C++
• These are declared in the same way as any method, in the
header file:
class Car {
private:
float price;
public:
Car();
void set_price (float p);
float query_price();
};

16. Constructors in C++
• Data fields in a C++ constructor are initialised like so:
Car::Car() :
price (200.0),
colour ("black") {
}
C++ Constructor Code
Note that the method here has no return type – this distinguishes it from other
kinds of methods in C++.

17. Constructors in C++
• Initialization lists may look like function calls.
• They’re not.
• The name of the entries in the list refer specifically to the name
of attributes only.
• You can make use of parameters passed to constructors in the
initialization lists also.
Car::Car(float p) :
price (p) {
}

18. Rules of Constructors
• If no constructors are defined by the developer, a default case
is provided.
• This is a zero parameter constructor with no attached
functionality.
• If any developer-defined constructor exists, the compiler will
not provide one.
• Even if the defined constructor has parameters.
• You should always provide a zero parameter constructor.
• We won’t talk about why – it’s just Good Practice.

19. Constructors
• The constructor method gets called whenever an object is
created.
• The compiler figures out which constructor should be called
by examining the parameter list.
• Only one of the constructors will be called.
• As far as C++ is concerned, they are all different methods even if
they share a name.

20. Default Parameters
• C++ Constructors (and methods) allow for default values to be
specified for parameters.
• This is a feature not available in Java.
• Default values are specified in the declaration only.
• Parameters with default values must come at the end of the
parameter list.

21. Declaring And Using A Default
Parameter
Declaration
Car (float, string = "bright green");
Using
int main() {
Car my_car (500.0);
cout << my_car.query_price() << endl;
return 1;
}

22. Why Use A Constructor?
• Constructors allow for the developer to ensure a minimum
level of data is contained within an object.
• Avoids the need to hard-code validation into data algorithms.
• You can assume that the objects you are working with will be
configured in some syntactically correct way.

23. Why Use A Constructor?
• In large multi-developer projects, while you can often assume
good faith, you cannot rely on it.
• You can provide documentation to say that every instantiation of
an object should be followed by calls to accessors. You can’t
ensure people follow it.
• Constructor methods allow you to enforce at compilation the
correctness of object configuration.
• The earlier in the development process errors are encountered,
the easier they are to fix.

24. Constructors in C++
• Constructors in specialised C++ classes work in a slightly
different way to in Java.
• Constructors cannot set the value in parent classes.
• This syntactically forbidden in C++.
• By default, the matching constructor in the derived class (child
class) will be called.
• Then the default construtor in the parent will be called.
• From our earlier example, the code will call the matching
constructor in ExtendedCar, and then the parameterless
constructor in Car.

25. Constructors in C++
• If we want to change that behaviour, we must indicate so to
the constructor.
• In our implementation for the constructor, we indicate which of
the constructors in the parent class are to be executed.
• We must handle the provision of the values ourselves.

26. Constructors in C++
class ExtendedCar : public Car {
private:
float mpg;
public:
ExtendedCar(
float cost = 100.0,
float mpg = 50.0);
void set_mpg (float f);
float query_mpg();
};
ExtendedCar::ExtendedCar (float cost, float mpg)
: Car(cost),
mpg (mpg) {
}

27. Summary
• Inheritance is a powerful tool for reusability in C++.
• We only touch on it in this module – no need to worry too much
about it.
• Constructor methods in a C++ program permit us to have
control over the starting values of data fields in our objects.
• Important and useful.