CSCI 261 - Programming Concepts (C++)Fall 2017 - Lab 7AQuick Links: Canvas | CS @ Mines | Cloud9 | Piazza | zyBooks |
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This lab is due by Tuesday, November 14, 2017 11:59 PM . ConceptsToday you will define what it means to be "Money"
to the machine. Focus on two main concepts for this assignment: (1) define classes with
public member variables and (2) define and implement constructors.
You will need to submit your class declaration
file, your class definition file, and your
main.cpp file for this
lab with Assignment 7..
Programming to the DomainWhen we say the word "domain" we really mean
"subject matter" or "topic." What then, does it mean to "program to the domain?"
Consider these two snippets of code:
string aliceTitle = "Alice in Wonderland";
int alicePages = 200; string favorite = "Great Mambo Chicken and the Transhuman Condition"; int favoritePage = 238; cout << aliceTitle << " " << alicePages; Notice how this first snippet above is really
about programming with strings and ints. Now let's look at a different snippet
of code:
Book alice("Alice in Wonderland", 200);
Book favorite("Great Mambo Chicken and the Transhuman Condition", 238); cout << alice << endl; Notice how this second snippet is really more
domain-specific. It's not about strings and ints, which have nothing to do
with books. This code is about books.
Whenever possible, empower yourself by writing
programs at higher levels of abstraction that are more specific to the domain
of the problem you are trying to solve or model. Wherever possible, create
programs about pipes, water, money, books, hydraulics, gates, bridges and
zombies -- not about chars, ints and strings.
ClassesWe are learning about C++ classes in lecture
this week. Note that classes are the primary mechanism in OO languages that
allow you to define more abstract datatypes (like tables, books, boxes, and
zombies) to the machine, and thereby create programs that use these
abstractions. Today, focus on your ability to define classes using a declaration
file (ending in
.h) and an definition file (ending in
.cpp).Perhaps the most tricky thing to remember
about the syntax for class declarations is the trailing semicolon at the end
of your class declaration's closing brace.
class Car {
}
class Car {
}; There you have it, the world's most simple
declaration of a Car, which a C++ compiler can actually comprehend.
Member Variables aka Data Members aka Attributes aka PropertiesRegardless of what you call them, when we want
to declare properties that objects can have, we use variables inside the class
declaration. For example:
class Car {
public: int horsepower; }; Now we are telling the computer, "Hey computer,
Cars are things with a property called horsepower, which is an integer." The
public: thing you see there means that programmers can access a
Car object's horsepower using the "dot-operator" (aka "member access operator").
For example:
Car junkTruck;
junkTruck.horsepower = 100; Assuming that every Car instance has a
horsepower property, we can access it just like the above snippet of code
illustrates.
ConstructorsConsider what the Car example above does. It
has merely declared the fact that there is such a thing as a Car. But how do
we "create" or instantiate Car objects? We do so through a family of special
functions that we call constructors. Realize that by default,
a C++ compiler provides a "hidden" default constructor when you do not define
one. In other words, with just the above class definition, we can instantiate
a Car.
Car junkTruck;
But what if you wanted to do the following?
Car junkTruck;
cout << junkTruck.horsepower; What is the value of
horsepower?
We don't know, as we didn't give it a default value. Although the C++ compiler
provides you the ability to create a default object, it's not smart enough to
initialize all the properties of your objects.Let's imagine that you'd like all Car objects
to have a default horsepower of 100 when instantiated. To do this, we just
have to define a default constructor ourselves.
Defining a default constructor requires two steps:
In the end, the file Car.h would look like this:
class Car {
public: Car(); // the prototype of the constructor "function" int horsepower; }; And Car.cpp would look like this:
#include "Car.h"
Car::Car() { horsepower = 100; } See the strange :: syntax? This is like saying,
"The Car class function called Car() is defined as..." followed by the function
body. Notice that constructors are just functions, but they're "special" in that:
Now that you have defined your own default
constructor, the next time you try the following, we would see 100 printed to
the console.
Car junkTruck;
cout << junkTruck.horsepower; In the example above, we instantiate a Car
called junkTruck, which is created via the default constructor. What does
the constructor do? It initializes the object's horsepower to 100.
How would you create a non-default constructor,
allowing us to assign a specific horsepower value upon instantiation?
Car goodTruck(400);
Solve the assignment below to practice how!
Instructions Download MoneyClass.zip
and add the files to your project. Notice that there are three
files in the zip file:
main.cpp (which you will not modify), Money.h (which is
where you declare your Money class), and Money.cpp (which is where
you define your Money class member functions). Declare and define a Money class such that the provided
main.cpp
will compile and execute appropriately. Specifically, your Money class should
have two
integers (called dollars and cents) and two constructors (one
default that sets the amount to 999.99 and one non-default). We suggest you first declare your class, and then add empty constructors (to
get the code to compile). Then write the "guts" for each constructor.
When you are done, the output should look like the following screen.
Who doesn't love money?
I have $999.99 You have $987.65 I have $12.34 more money than you! In your first class declaration (today's lab),
you can let your data members be public. Starting
next class, we'll want to keep data members private.
Lab Submission
You will submit your solution to this lab with the rest of A7.
Detailed instructions for doing this are posted in Assignment 7.
This lab is due by Tuesday, November 14, 2017 11:59 PM . | |
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Last Updated: 11/05/17 21:23
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