C++ Preprocessing Directives

Introduction

Directive	Description
#include	Supports header file inclusion
#if	Enables conditional compilation
#else	else for #if
#elif	Equivalent to #else #if
#endif	Marks the end of an #if directive
#define	Defines an identifier
#undef	Deletes an identifier
#if defined (or #ifdef)	Does something if an identifier defined
#if !defined (or #ifndef)	Does something if an identifier is not defined
#line	Redefines the current line number and/or filename
#error	Outputs a compile-time error message and stop the compilation.
#pragma	Offers machine-specific features while retaining overall C++ compatibility

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