A C preprocessor macro to test whether a macro parameter has a value.

if_value.h

//
// if_value.h
// Brandon Azad
//
// Public domain
//

#ifndef IF_VALUE

/*
 * IF_VALUE(_x_)(_y_)(_z_)
 *
 * Description:
 * 	If _x_ is not empty, produces _y_, else produces _z_. Do not use or redefine macros
 * 	beginning with _IF_VALUE_.
 */

// _IF_VALUE__SECOND() returns the second argument.
#define _IF_VALUE__SECOND(_a, _b, ...)		_b
// _IF_VALUE__SECOND_2() does the same as _IF_VALUE__SECOND() after an additional round of
// parameter expansion.
#define _IF_VALUE__SECOND_2(_a, _b, ...)	_IF_VALUE__SECOND(_a, _b, __VA_ARGS__)
// _IF_VALUE__CHECK__IF_VALUE__IS_EMPTY() is used by _IF_VALUE__CHECK to test whether the argument
// is _IF_VALUE__IS_EMPTY and return _IF_VALUE__IS_EMPTY if so.
#define _IF_VALUE__CHECK__IF_VALUE__IS_EMPTY()	~, _IF_VALUE__IS_EMPTY
// _IF_VALUE__CHECK() produces _IF_VALUE__IS_EMPTY if _first is _IF_VALUE__IS_EMPTY and
// _IF_VALUE__NOT_EMPTY otherwise.
#define _IF_VALUE__CHECK(_first, ...)		_IF_VALUE__SECOND_2(_IF_VALUE__CHECK_ ## _first (), _IF_VALUE__NOT_EMPTY, ~)
// _IF_VALUE__CHECK_2() does the same as _IF_VALUE__CHECK() after an additional round of parameter
// expansion.
#define _IF_VALUE__CHECK_2(_first, ...)		_IF_VALUE__CHECK(_first, __VA_ARGS__)
// _IF_VALUE__IS_EMPTY takes 2 groups of parentheses and returns the unwrapped value of the first
// group.
#define _IF_VALUE__IS_EMPTY(...)		_IF_VALUE__IS_EMPTY_1
#define _IF_VALUE__IS_EMPTY_1(...)		__VA_ARGS__
// _IF_VALUE__NOT_EMPTY takes 2 groups of parentheses and returns the unwrapped value of the second
// group.
#define _IF_VALUE__NOT_EMPTY(...)		__VA_ARGS__ _IF_VALUE__NOT_EMPTY_1
#define _IF_VALUE__NOT_EMPTY_1(...)		/* nothing */
// IF_VALUE() produces _IF_VALUE__IS_EMPTY if _maybe_empty is empty and _IF_VALUE__NOT_EMPTY
// otherwise.
#define IF_VALUE(_maybe_empty)	_IF_VALUE__CHECK_2(_IF_VALUE__CHECK _maybe_empty (_IF_VALUE__IS_EMPTY, ~), ~)

#endif