Directives and the Preprocessor
performs preliminary operations on C++ files before they are passed
to the compiler. These operations include inserting source code from
files, defining constants, changing your source code and other operations.
in the source file tell the preprocessor to perform specific actions.
There are quite a few directives the preprocessor recognizes; here are
The first character is the # symbol,
which is a signal to the preprocessor. Each time you start your compiler,
the preprocessor is run. 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 able to add and
remove code from your source file using the #include
directive. This is an instruction that tells the preprocessor to include
code from a particular file. As an example, consider the HelloWorld
program which uses the following directive:-
This tells the preprocessor to include
code from the file iostream.h (Input-Output-Stream).
This file contains declarations for functions that the program needs
to use, such as the cout function.
The cout function helps you
to write to the screen.
The effect of the directive is as
if you had typed in the code inside iostream.h into your
own file. In other words the #include
directive says, "What follows is a filename; find that file and
read it in right here."
Of course, the preprocessor has to
find the file first. This is what the angle brackets around the filename
are for. They tell the preprocessor to look in the usual folder that
holds all the .H files for your compiler.
Normally this will be the C:\Program Files\Microsoft Visual Studio\VC98\Include
directive is used to to define a constant in your program.
Here is an example:-
#define PI 3.141
Wherever the constant name PI
appears in your source file, the preprocessor replaces it by its value.
When the compiler runs, the compiler will only see the value 3.141
in your code, not PI. Every
PI is just replaced by its
We will be covering constants in more
detail later on in the course.
Carry out the following exercises:-
- Open your helloWorld program. Then locate the following
Remove the whole of the line from your code. Try
and compile your program by selecting Compile or Build
HelloWorld.exe from the Build menu. What error
message do you get? You should get:-
This is because the line you removed told VC++ to include
the a file called iostream.h that comes with VC++.
You need this file to be able to use the cout
function which sends output to the command prompt console. We shall
discuss this in a later lesson. Make sure you type the line back in.
'cout' : undeclared