Preprocessing Directives - C++ Preprocessor

Introduction

Defining Preprocessing Identifiers

The general form of the #define preprocessing directive is:

#define identifier sequence_of_characters

The following code defines PI to be an alias for a sequence of characters that represents a numerical value:

#define PI 3.14159265

The #define directive is often used to define symbolic constants in C but don't do this in C++.

In C++, do it this way.

const long double pi {3.14159265L};

Define it with external linkage:

extern const long double pi {3.14159265L};

Here's another example:

#define BLACK WHITE

If you don't specify a substitution string for an identifier, then the identifier will be replaced by an empty string.

For example:

#define VALUE

Undefining an Identifier

You can negate a previously defined VALUE identifier with this directive:

#undef VALUE

Example,

#define PI 3.142
// All occurrences of PI in code from this point will be replaced by 3.142

#undef PI
// PI is no longer defined from here on so no substitutions occur.
// Any references to PI will be left in the code.

Including Header Files

Example,

#include <iostream>
#include "myheader.h"

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Related Tutorials

• Reducing #includes with Forward Class Declarations
• Including an Inline File
• Use #ifdef to do condition compilation
• Standard Preprocessing Macros
• Use #if to check value