1.6. The First Program

Traditionally the first program people write in a new language is called “Hello, World.” because all it does is print the words “Hello, World.” In C++, this program looks like this:

The “Hello, World!” program is a great place to start learning a new language. Observe the program structure below.

Some people judge the quality of a programming language by the simplicity of the “Hello, World.” program. By this standard, C++ does reasonably well.

The fist line, which starts with #include is simply a way to use features that are not “built in” to the language, but are available for you to use. Soon you’ll learn how to create your own, but for now we are using code from the Standard Library. All of the facilities provided by the standard library have the prefix std::.

The second line begins with //, which indicates that it is a comment. A comment is a bit of English text that you can put in the middle of a program, usually to explain what the program does. When the compiler sees a //, it ignores everything from there until the end of the line.

In the third line, you can ignore the word int for now, but notice the word main. main is a special name that indicates the place in the program where execution begins. When the program runs, it starts by executing the first statement in main and it continues, in order, until it gets to the last statement, and then it quits.

There is no limit to the number of statements that can be in main, but the example contains only one. It is a basic output statement, meaning that it outputs or displays a message on the screen.

cout is an object available from the C++ Standard Library which allows you to send output to the screen. The std:: prefix means the facility we are using is part of the standard library. The symbol << is an operator that you apply to cout and a string, and that causes the string to be displayed.

endl is an object that represents the end of a line. When you send an endl to cout, it causes the cursor to move to the next line of the display. The next time you output something, the new text appears on the next line.

Like all statements, the output statement ends with a semi-colon (;).

Warning

Failure to terminate a statement with a semicolon will result in a syntax (compile) error.

There are a few other things you should notice about the syntax of this program. First, C++ uses curly-braces ({ and }) to group things together. In this case, the output statement is enclosed in curly-braces, indicating that it is inside the definition of main. Also, notice that the statement is indented, which helps to show visually which lines are inside the definition.

As I mentioned, the C++ compiler is a real stickler for syntax. If you make any errors when you type in the program, chances are that it will not compile successfully. For example, if you misspell iostream, you might get an error message like the following:

hello.cpp:1: oistream: No such file or directory

There is a lot of information on this line, but it is presented in a dense format that is not easy to interpret. A more friendly compiler might say something like:

“On line 1 of the source code file named hello.cpp, you tried to include a header file named oistream. I didn’t find anything with that name, but I did find something named iostream. Is that what you meant, by any chance?”

Unfortunately, few compilers are so accommodating. The compiler is not actually smart, and in most cases the error message you get will be only a hint about what is wrong. It will take some time to gain facility at interpreting compiler messages.

Nevertheless, the compiler can be a useful tool for learning the syntax rules of a language.

Try This!

Starting with the working ‘hello world’ program above, modify it in various ways and see what happens. If you get an error message, try to remember what the message says and what caused it, so if you see it again in the future you will know what it means.

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