Sams Teach Yourself C++ in 21 Days

LISTING2.1 HELLO.cppDemonstrates the Parts of a C++ Program

1: #include <iostream>

2:

3: int main()

4: {

5: std::cout << “Hello World!\n”;

6: return 0;

7: }

Hello World!

On line 1, the file iostreamis included into the current file.

Here’s how that works: The first character is the #symbol, which is a signal to a program

called the preprocessor. Each time you start your compiler, the preprocessor is run first.

The preprocessor reads through your source code, looking for lines that begin with the

pound symbol (#) and acts on those lines before the compiler runs. The preprocessor is

discussed in detail on Day 21, “What’s Next.”

The command #includeis a preprocessor instruction that says, “What follows is a file-

name. Find that file, read it, and place it right here.” The angle brackets around the file-

name tell the preprocessor to look in all the usual places for this file. If your compiler is

set up correctly, the angle brackets cause the preprocessor to look for the file iostream

in the directory that holds all the include files for your compiler. The file iostream

(Input-Output-Stream) is used by cout, which assists with writing to the console. The

effect of line 1 is to include the file iostreaminto this program as if you had typed it in

yourself.

OUTPUT

26 Day 2

ANALYSIS

The preprocessor runs before your compiler each time the compiler is

invoked. The preprocessor translates any line that begins with a pound sym-

bol (#) into a special command, getting your code file ready for the compiler.

NOTE

Not all compilers are consistent in their support for #includes that omit the

file extension. If you get error messages, you might need to change the

include search path for your compiler, or add the extension to the #include.

NOTE