#ifdef and #ifndef
Go Up to Conditional Compilation Directives (C++)
Syntax
#ifdef identifier #ifndef identifier
Description
The #ifdef and #ifndef directives are the same as the #if directive with defined (identifier) operator:
#if defined identifier #if !defined identifier
You can use the #ifdef and #ifndef directives instead of the #if defined identifier expressions. However, the #if defined identifier expressions are preferred. The #ifdef and #ifndef directives are provided only for compatibility with previous versions of the language.
The #ifdef and #ifndef conditional directives let you test whether an identifier is currently defined or not. That is, whether a previous #define command has been processed for that identifier and is still in force. The line
#ifdef identifier
has exactly the same effect as
#if 1
if identifier is currently defined, and the same effect as
#if 0
if identifier is currently undefined.
An #ifndef evaluates "true" for the "not-defined" identifier, so the line
#ifndef identifier
has exactly the same effect as
#if 0
if identifier is currently defined, and the same effect as
#if 1
if identifier is currently undefined.
The syntax thereafter follows that of the #if, #elif, #else, and #endif.
An identifier defined as NULL is considered to be defined.