#ifdef - #ifndef - #elseif - #elif - #else - #endif

  The #ifdef, #ifndef, #elseif, #elif, #else, and #endif are the logical directives of the preprocessor. These logical operations determine the fate of information processed the preprocessor. The syntax of the conditional directives are: #ifdef Name0 lines to evaluate if Name0 is defined #elseif Name1 lines to evaluate if Name1 is defined, and Name0 is Not defined ... #elif NameN lines to evaluate if NameN is defined, and Name0-Name(N-1) is Not defined #else lines to evaluate if Name0-NameN are Not defined #endif

and

#ifndef Name0 lines to evaluate if Name0 is Not defined #elseif Name1 lines to evaluate if Name1 is defined, and Name0 is defined #else lines to evaluate if Name0 is defined and Name1 is Not defined #endif

The expression #ifdef Name evaluates to true in Name has been #defined. In this case (true), the input file lines are processed until a #elseif, #else, or #endif directive is encountered. The #elseif, #else, and #endif directives are valid only following a #ifdef directive. If the #ifdef directive evaluated to be true, t hen these directives (#elseif, #else, and #endif) stop processing of the input lines until a #endif directive is encounter. Thus, the code

/* simple_ifdef.inp */
#define TEST1
#define TEST2
#ifdef TEST1
test1 is defined, test2 and test3 are unchecked
#elseif TEST2
test1 is not defined, test2 is defined, test3 is unchecked
#elseif TEST3
test1 and test2 are not defined, test3 is defined
#else
test1, test2, test3 are not defined.
#endif
/* check for TEST2 alone */
#ifdef TEST2
test2 is defined
#endif

evaluates to be

test1 is defined, test2 and test3 are unchecked
test2 is defined

If an #ifdef is evaluated to be false, then the #elseif directive is processed as if it was an #ifdef directive. If all #elseif directives evaluate to be false, the #else directive is processed if it exists. Input is evaluated until an #endif directive is encountered. A #endif directive ends the #ifdef processing.

#ifdef directives can be nested, however, all directives in a given logical level must exist in a single file.

The following sample code showing use of the #ifdef directive chain with nested #ifdefs.

/* ifdef_example.inp */
#define TEST1  100 // define a variable.
#define TEST2
/* process the input based on the what was defined above */
#ifdef TEST1
TEST1 (test1) is defined 
#ifdef TEST2   // another check (nested within the first)
The Value TEST2 (test2) is defined
#else          // else for #ifdef TEST2
TEST1 (test1) is defined and TEST2 is not defined
#endif        // #endif for #ifdef TEST2
#elseif TEST2
TEST1 is NOT Defined,  and TEST2 (test2) is defined
#else
TEST1 and TEST2 are not defined
#endif

It evaluates to be equal to

100 (test1) is defined 
The Value TEST2_DEFINED_WITHOUT_A_VALUE (test2) is defined

The expression #ifndef Name evaluates to true in Name has not been #defined. Processing continues similar to an ifdef directive.

The #elif is functionally equivalent to the #elseif directive. These directives are interchangeable.