Hour 2. Organizing the Parts of a Program

Although it recently turned 30, the C++ programming language has aged a lot better than some other things that came out in the late 1970s. Unlike disco, oil embargoes, shag carpet, and avocado-colored refrigerators, C++ is still in vogue today. It remains a world-class programming language.

The reason for its surprising longevity is that C++ makes it possible to create fast executing programs with a small amount of code that can run on a variety of computing environments. Today’s C++ programming tools enable the creation of complex and powerful applications in commercial, business, and open source development.

Reasons to Use C++

During the seven decades of the computing age, computer programming languages have undergone a dramatic evolution. C++ is considered to be an evolutional improvement of a language called C that was introduced in 1972.

The earliest programmers worked with the most primitive computer instructions: machine language. These instructions were represented by long strings of 1s and 0s. Assemblers were devised that could map machine instructions to human-readable and manageable commands such as ADD and MOV.

The instructions that make up a computer program are called its source code.

In time, higher-level languages were introduced such as BASIC and COBOL. These languages made it possible for programmers to begin to craft programs using language closer to actual words and sentences, such as Let Gpa = 2.25. These instructions were translated back into machine language by tools that were called either interpreters or compilers.

An interpreter-based language such as BASIC translates a program as it reads each line, acting on each instruction.

A compiler-based language translates a program into what is called object code through a process called compiling. This code is stored in an object file. Next, a linker transforms the object file into an executable program that can be run on an operating system.

Because interpreters read the code as it is written and execute the code on-the-fly, they’re easy for programmers to work with. Compilers require the more inconvenient extra steps of compiling and linking programs. The benefit to this approach is that the programs run significantly faster than programs run by an interpreter.

For many years, the principal goal of computer programmers was to write short pieces of code that would execute quickly. Programs needed to be small because memory was expensive, and they needed to be fast because processing power also was expensive. As computers have become cheaper, faster, and more powerful and the cost and capacity of memory has fallen, these priorities diminished in importance.

Today, the greatest expense in programming is the cost of a programmer’s time. Modern languages such as C++ make it easier to produce well-written, easy-to-maintain programs that can be extended and enhanced.

Styles of Programming

As programming languages have evolved, languages have been created to cater to different styles of programming.

In procedural programming, programs are conceived of as a series of actions performed on a set of data. Structured programming was introduced to provide a systematic approach to organizing these procedures and managing large amounts of data.

The principle idea behind structured programming is to divide and conquer. Take a task that needs to be accomplished in a program, and if it is too complex, break it down into a set of smaller component tasks. If any of those tasks are still too complicated, break them down into even smaller tasks. The end goal is tasks that are small and self-contained enough to be easily understood.

As an example, pretend you’ve been asked by this publisher to write a program that tracks the average income of its team of enormously talented and understatedly good-looking computer book authors. This job can be broken down into these subtasks:

1. Find out what each author earns.

2. Count how many authors the publisher has.

3. Total all their income.

4. Divide the total by the number of authors.

Totaling the income can be broken down into the following:

1. Get each author’s personnel record.

2. Access the author’s book advances and royalties.

3. Deduct the cost of morning coffee, corrective eyewear and therapy.

4. Add the income to the running total.

5. Get the next author’s record.

In turn, obtaining each author’s record can be broken down into these subtasks:

1. Open the file folder of authors.

2. Go to the correct record.

3. Read the data from disk.

Although structured programming has been widely used, some drawbacks attach to the approach. The separation of data from the tasks that manipulate the data becomes harder to work with as the amount of data grows. The more things that must be done with data, the more confusing a program becomes.

Procedural programmers often find themselves reinventing new solutions to old problems instead of producing reusable programs. The idea behind reusability is to build program components that can be plugged into programs as needed. This approach is modeled after the physical world, where devices are built out of individual parts that each perform a specific task and have already been manufactured. A person designing a bicycle doesn’t have to create a brake system from scratch.

Instead, she can incorporate an existing brake into the design and take advantage of its functionality.

Before the introduction of object-oriented programming, there was no similar option for a computer programmer.

C++ and Object-Oriented Programming

The essence of object-oriented programming is to treat data and the procedures that act upon the data as a single object—a self-contained entity with an identity and characteristics of its own.

The C++ language fully supports object-oriented programming, including three concepts that have come to be known as the pillars of object-oriented development: encapsulation, inheritance, and polymorphism.

Encapsulation

When the aforementioned bike engineer creates a new bicycle, she connects together component pieces such as the frame, handlebars, wheels, and a headlight. Each component has certain properties and can accomplish certain behaviors. She can use the headlight without understanding the details of how it works, as long as she knows what it does.

To achieve this, the headlight must be self-contained. It must do one well-defined thing and it must do it completely. Accomplishing one thing completely is called encapsulation.

All the properties of the headlight are encapsulated in the headlight object. They are not spread out through the bicycle.

C++ supports the properties of encapsulation through the creation of user-defined types called classes. A well-defined class acts as a fully encapsulated entity that is used as an entire unit or not at all. The inner workings of the class should be hidden on the principle that the programs that use a well-defined class do not need to know how the class works. They only need to know how to use it. You learn how to create classes in Hour 8, “Creating Basic Classes.”

Inheritance and Reuse

Now we’re starting to learn a little more about our bike engineer. Let’s call her Penny Farthing. Penny needs her new bicycle to hit the market quickly—she has run up enormous gambling debts to people who are not known for their patience.

Because of the urgency, Penny starts with the design of an existing bicycle and enhances it with cool new add-ons like a cup holder and mileage counter. Her new enhanced bicycle is conceived as a kind of bicycle with added features. She reused all the features of a regular bicycle while adding capabilities to extend its utility.

C++ supports the idea of reuse through inheritance. A new type can be declared that is an extension of an existing type. This new subclass is said to derive from the existing type. Penny’s bicycle is derived from a plain old bicycle and thus inherits all its qualities but adds additional features as needed. Inheritance and its application in C++ are discussed in Hour 16, “Extending Classes with Inheritance.”

Polymorphism

As its final new selling point, Penny Farthing’s Amazo-Bicycle™ behaves differently when its horn is squeezed. Instead of honking like a sickly goose, it sounds like a car when lightly pressed and roars like a foghorn when strongly squashed. The horn does the right thing and makes the proper sound based on how it is used by the bicycle’s rider.

C++ supports this idea that different objects do the right thing through a language feature called function polymorphism and class polymorphism. Polymorphism refers to the same thing taking many forms, and is discussed during Hour 17, “Using Polymorphism and Derived Classes.”

You will learn the full scope of object-oriented programming by learning C++. These concepts will become familiar to you by the time you’ve completed the full 24-hour ride and begun to develop your own C++ programs.

You won’t learn how to design bicycles or get out of gambling debt.

The Parts of a Program

The program you created during the first hour, Motto.cpp, contains the basic framework of a C++ program. Listing 2.1 reproduces the source code of this program so that it can be explored in more detail.

When typing this program into your programming editor, remember not to include the line numbers in the listing. They are included solely for the purpose of referring to specific lines in this book.

Listing 2.1 The Full Text of Motto.cpp

---

1:  #include <iostream>
2:
3:  int main()
4:  {
5:      std::cout << "Solidum petit in profundis! ";
6:      return 0;
7:  }

---

This program produces a single line of output, the motto of Aarhus University:

Solidum petit in profundis!

On line 1 of Listing 2.1 a file named iostream is included in the source code. This line causes the compiler to act as if the entire contents of the file were typed at that place in Motto.cpp.

Preprocessor Directives

A C++ compiler’s first action is to call another tool called the preprocessor that examines the source code. This happens automatically each time the compiler runs.

The first character in line 1 is the # symbol, which indicates that the line is a command to be handled by the preprocessor. These commands are called preprocessor directives. The preprocessor’s job is to read source code looking for directives and modify the code according to the indicated directive. The modified code is fed to the compiler.

The preprocessor serves as an editor of code right before it is compiled. Each directive is a command telling that editor what to do.

The #include directive tells the preprocessor to include the entire contents of a designated filename at that spot in a program. C++ includes a standard library of source code that can be used in your programs to perform useful functionality. The code in the iostream file supports input and output tasks such as displaying information onscreen and taking input from a user.

The < and > brackets around the filename iostream tell the preprocessor to look in a standard set of locations for the file. Because of the brackets, the preprocessor looks for the iostream file in the directory that holds header files for the compiler. These files also are called include files because they are included in a program’s source code.

The full contents of iostream are included in place of line 1.

The contents of the file iostream are used by the cout command in line 5, which displays information to the screen.

There are no other directives in the source code, so the compiler handles the rest of Motto.cpp.

Source Code Line by Line

Line 3 begins the actual program by declaring a function named main(). Functions are blocks of code that perform one or more related actions. Functions do some work and then return to the spot in the program where they were called.

Every C++ program has a main() function. When a program starts, main() is called automatically.

All functions in C++ must return a value of some kind after their work is done. The main() function always returns an integer value. Integers are specified using the keyword int.

Functions, like other blocks of code in a C++ program, are grouped together using the brace marks { and }. All functions begin with an opening brace { and end with a closing brace }.

The braces for the main() function of Motto.cpp are on lines 4 and 7, respectively. Everything between the opening and closing braces is part of the function.

In line 5, the cout command is used to display a message on the screen. The object has the designation std:: in front of it, which tells the compiler to use the standard C++ input/output library. The details of how this works are too complex for this early hour and likely will cause you to throw the book across the room if introduced here. For the safety of others in your vicinity, they are explained in a later hour. For now, treat std::cout as the name of the object that handles output in your programs and std::cin as the object that handles user input.

The reference to std::cout in line 5 is followed by <<, which is called the output redirection operator. Operators are characters in lines of code that perform an action in response to some kind of information. The << operator displays the information that follows it on the line. In line 5, the text “Solidum petit in profundis! ” is enclosed within double quotes. This displays a string of characters on the screen followed by a special character specified by , a newline character that advances the program’s output to the beginning of the next line.

On line 6, the program returns the integer value 0. This value is received by the operating system after the program finishes running. Typically, a program returns the value 0 to indicate that it ran successfully. Any other number indicates a failure of some kind.

The closing brace on line 7 ends the main() function, which ends the program. All of your programs use the basic framework demonstrated by this program.

Comments

As you are writing your own programs, it will seem perfectly clear to you what each line of the source code is intended to accomplish. But as time passes and you come back to the program later to fix a bug or add a new feature, you often will find yourself completely mystified by your own work.

To avoid this predicament and help others understand your code, you can document your source code with comments. Comments are lines of text that explain what a program is doing. The compiler ignores them, so they are strictly for benefit of humans reading the code.

There are two types of comments in C++. A single-line comment begins with two slash marks (//) and causes the compiler to ignore everything that follows the slashes on the same line. Here’s an example:

// The next line is a kludge (ugh!)

A multiple-line comment begins with the slash and asterisk characters (/*) and ends with the same characters reversed (*/). Everything within the opening /* and the closing */ is a comment, even if it stretches over multiple lines. If a program contains a /* that is not followed by a */ somewhere, that’s an error likely to be flagged by the compiler. Here’s a multiline comment:

/* This part of the program doesn't work very well. Please remember to
   fix this before the code goes live –– or else find a scapegoat you can
   blame for the problem. The new guy Curtis would be a good choice. */

In the preceding comment, the text on the left margin is lined up to make it more readable. This is not required. Because the compiler ignores everything within the /* and */, anything can be put there—grocery lists, love poems, secrets you’ve never told anybody in your life, and so on.

The next project that you create includes both kinds of comments. Write lots of comments in your programs. The more time spent writing comments that explain what’s going on in code, the easier that code is to work on weeks, months or even years later.

Functions

The main() function is unusual among C++ functions because it’s called automatically when a program begins.

A program is executed line by line in source code, beginning with the start of main(). When a function is called, the program branches off to execute the function. After the function has done its work, it returns control to the line where the function was called. Functions may or may not return a value, with the exception of main(), which always returns an integer.

Functions consist of a header and a body. The header consists of three things:

• The type of data the function returns

• The function’s name

• The parameters received by the function

The function’s name is a short identifier that describes its purpose.

When a function does not return a value, it uses data type void, which means the same thing as nothing.

Arguments are data sent to the function that control what it does. These arguments are received by the function as parameters. A function can have zero, one, or more parameters. The next program that you create has a function called add() that adds two numbers together. Here’s how it is declared:

int add(int x, int y)
{
    // body of function goes here
}

The parameters are organized within parentheses marks as a list separated by commas. In this function, the parameters are integers named x and y.

The name of a function, its parameters and the order of those parameters is called its signature. Like a person’s signature, the function’s signature uniquely identifies it.

A function with no parameters has an empty set of parentheses, as in this example:

int getServerStatus()
{
    // body of function here
}

Function names cannot contain spaces, so the getServerStatus() function capitalizes the first letter of each word after the first one. This naming convention is common among C++ programmers and adopted throughout this book.

The body of a function consists of an opening brace, zero or more statements, and a closing brace. A function that returns a value uses a return statement, as you’ve seen in the Motto program:

return 0;

The return statement causes a function to exit. If you don’t include at least one return statement in a function, it automatically returns a void at the end of the function’s body. This void must be specified as the function’s return type.

Using Arguments with Functions

The Calculator.cpp program in Listing 2.2 fleshes out the aforementioned add() function, using it to add two pairs of numbers together and display the results. This program demonstrates how to create a function that takes two integer arguments and returns an integer value.

Listing 2.2 The Full Text of Calculator.cpp

---

 1: #include <iostream>
 2:
 3: int add(int x, int y)
 4: {
 5:     // add the numbers x and y together and return the sum
 6:     std::cout << "Running calculator ... ";
 7:     return (x+y);
 8: }
 9:
10: int main()
11: {
12:     /* this program calls an add() function to add two different
13:        sets of numbers together and display the results. The
14:        add() function doesn't do anything unless it is called by
15:        a line in the main() function. */
16:     std::cout << "What is 867 + 5309? ";
17:     std::cout << "The sum is " << add(867, 5309) << " ";
18:     std::cout << "What is 777 + 9311? ";
19:     std::cout << "The sum is " << add(777, 9311) << " ";
20:     return 0;
21: }

---

This program produces the following output:

What is 867 + 5309?
Running calculator ...
The sum is 6176

What is 777 + 9311?
Running calculator ...
The sum is 10088

The Calculator program includes a single line comment on line 5 and a multiline comment on lines 12–15. All comments are ignored by the compiler.

The add() function takes two integer parameters named x and y and adds them together in a return statement (lines 3–8).

The program’s execution begins in the main() function. The first statement in line 16 uses the object std::cout and the redirection operator << to display the text “What is 867 + 5309?” followed by a newline.

The next line displays the text “The sum is” and calls the add() function with the arguments 867 and 5309. The execution of the program branches off to the add() function, as you can tell in the output by the text “Running calculator....”

The integer value returned by the function is displayed along with two more newlines.

The process repeats for a different set of numbers in lines 18–19.

The formula (x+y) is an expression. You learn how to create your own in Hour 4, “Using Expressions, Statements, and Operators.”

Summary

During this hour, you were shown how C++ evolved from other styles of computer languages and embraced a methodology called object-oriented programming. This methodology has been so successful in the world of computing that the language remains as contemporary today as it did when it was invented in 1979.

The mullet haircut I sported in college did not fare as well.

In the two programs that you developed during this hour, you made use of three parts of a C++ program: preprocessor directives, comments, and functions.

All the programs that you will create in C++ employ the same basic framework as the Motto and Calculator programs. They will just become more sophisticated as they make use of more functions, whether you write them from scratch or call functions from header files included with the #include directive.

Q&A

Q. What does the # character do in a C++ program?

A. The # symbol signals that the line is a preprocessor directive, a command that is handled before the program is compiled. The #include directive includes the full text of a file at that position in the program. The compiler never sees the directive. Instead, it acts as if the contents of the file were typed in with the rest of the source code.

Q. What is the difference between // comments and /* style comments?

A. The comments that start with // are single-line comments that end with the end of the line on which they appear. The /* comments are multiline comments that don’t end until a */ is encountered. The end of a function won’t even cause a multiline comment to be ended. You must put in the closing */ mark or the compiler will fail with an error.

Q. What’s the difference between function arguments and function parameters?

A. The terms are related to the same process—calling a function by providing it with one or more data types the function will use to accomplish its work. Arguments are the information sent to the function. Parameters are the same information received inside the function. You call a function with arguments. Within a function, those arguments are received as parameters.

Q. What is a kludge?

A. A kludge is an ugly solution to a problem that’s intended to be replaced later with something better. The term was popularized by Navy technicians, computer programmers and aerospace engineers and spread to other technical professions.

In a computer program a kludge is source code that works but would have been designed better if there had been enough time. Kludges have a tendency to stick around a lot longer than expected.

The astronauts on the Apollo 13 mission created one of the greatest kludges of all time: a system cobbled together from duct tape and socks that filtered carbon dioxide from the air on the spacecraft and helped them make it back to Earth.

The first known usage of the term was in a 1962 article in Datamation magazine by Jackson W. Granholm, who gave it an elegant definition that has stood the test of time: “An ill-assorted collection of poorly-matching parts, forming a distressing whole.”

Workshop

Now that you’ve learned about some of the pieces of a C++ program, you can answer a couple of questions and complete a couple of exercises to firm up your knowledge.

Quiz

1. What data type does main return?

A. void

B. int

C. It does not return a type.

2. What do the braces do in a C++ program?

A. Indicate the start and end of a function

B. Indicate the start and end of a program

C. Straighten the program’s teeth

3. What is not part of a function’s signature?

A. Its name

B. Its arguments

C. Its return type

Answers

1. B. main returns an int (integer).

2. A. Braces mark the start and end of functions and other blocks of code you learn about in upcoming hours.

3. C. A function signature consists of its name, parameters, and the precise order of those parameters.

Activities

1. Rewrite the Motto program to display the Aarhus University motto in a function.

2. Rewrite the Calculator program to add a third integer called z in the add() function and call this function with two sets of three numbers.

To see solutions to these activities, visit this book’s website at http://cplusplus.cadenhead.org.