C++ examples for Preprocessor:Directive
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 |
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
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.
Example,
#include <iostream> #include "myheader.h"