The #ifdef and #ifndef directives perform the same task as the #if directive when it is used with defined( identifier ).
Syntax
#ifdef identifier
#ifndef identifier
is 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:
// PROG.CPP
#ifndef test // These three statements go in your source code.
#define final
#endif
CL /Dtest prog.cpp // This is the command for compilation.
END Microsoft Specific