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:

Some people judge the quality of a programming language by the simplicity of the "Hello, World." program. By this standard, C++ does reasonably well. Even so, this simple program contains several features that are hard to explain to beginning programmers. For now, we will ignore some of them, like the first line.

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 void 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 a special object provided by the system to allow you to send output to the screen. The symbol << is an operator that you apply to cout and a string, and that causes the string to be displayed.

endl is a special symbol 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 (;).

There are a few other things you should notice about the syntax of this program. First, C++ uses squiggly-braces ({ and }) to group things together. In this case, the output statement is enclosed in squiggly-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.

At this point it would be a good idea to sit down in front of a computer and compile and run this program. The details of how to do that depend on your programming environment, but from now on in this book I will assume that you know how to do it.

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:

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.h. I didn't find anything with that name, but I did find something named iostream.h. Is that what you meant, by any chance?"

Unfortunately, few compilers are so accomodating. The compiler is not really very 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. Starting with a working program (like hello.cpp), 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.