Preprocessor – the #else Directive

#else is one of five preprocessor selection statements allowing selection of alternative sections of code for compilation. The other four selection statements are: #ifdef, #ifndef, #if, and #elif.

Behaviour of this preprocessor directive is the same for both C and C++ compilers.

Purpose

The #else directive provides a final alternative for a preprocessor selection block. If all previous selection options fail, then the code found between the #else and #endif is compiled.

Format

#if or #ifdef or #ifndef

preprocessor or code statements

#elif controlling_expression (optional)

preprocessor or code statements

#else

preprocessor or code statements

#endif

All preprocessor directives begin with the # symbol. It must be the first character on the line or the first character on the line following optional white space.

Some early compilers flagged an error if # was not the first character on the line.

Spaces or tabs are permitted between the # and else, but not escape characters or other symbols or macros. The preprocessor removes white space and concatenates the # and else together.

If anything follows the #else directive (other than white space) then the program is malformed.

The following are valid uses:

#else
#   else
# /* comments are white space */ else

The following are invalid uses:

// #\ is not a valid preprocessor directive
# \t else
// #" is not a valid preprocessor directive
# "" else
// malformed because only white space may follow #else
#else MY_MACRO

Use

The #else must appear as the last alternative in a preprocessor selection block. If all the preceding selection options fail, then the code contained between the #else and closing #endif is compiled.

The statements following the #else may be language statements or preprocessor statements. There is no limit on the nesting of preprocessor directives or statements.

 #ifdef MY_MACRO
.
.
.
// these statements will only be compiled if MY_MACRO exists
.
.
.
#else
.
.
.
// these statements will only be compiled if MY_MACRO does not exist
.
.
.
#endif

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