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. 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 two lines.
The third 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
//, it ignores everything from there until the end of the line.
In the fourth line, you can ignore the word
int for now, but notice the
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
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
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 (
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 squiggly-braces (
}) to group things
together. In this case, the output statement is enclosed in
squiggly-braces, indicating that it is inside the definition of
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
might get an error message like the following:
hello.cpp:1: oistream.h: 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.h. 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 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 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.