#ifdef and #ifndef Directives (C/C++)


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The #ifdef and #ifndef directives perform the same task as the #if directive when it is used with defined( identifier ).

#ifdef identifier  
#ifndef identifier  
// equivalent to  
#if defined identifier  
#if !defined identifier  

You can use the #ifdef and #ifndef directives anywhere #if can be used. The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined, and it is equivalent to #if 0 when identifier has not been defined or has been undefined with the #undef directive. These directives check only for the presence or absence of identifiers defined with #define, not for identifiers declared in the C or C++ source code.

These directives are provided only for compatibility with previous versions of the language. The defined( identifier ) constant expression used with the #if directive is preferred.

The #ifndef directive checks for the opposite of the condition checked by #ifdef. If the identifier has not been defined (or its definition has been removed with #undef), the condition is true (nonzero). Otherwise, the condition is false (0).

Microsoft Specific

The identifier can be passed from the command line using the /D option. Up to 30 macros can be specified with /D.

This is useful for checking whether a definition exists, because a definition can be passed from the command line. For example:

// ifdef_ifndef.CPP  
// compile with: /Dtest /c  
#ifndef test  
#define final  

END Microsoft Specific

Preprocessor Directives