What is the purpose of the unary plus (+) operator in C?

Question

In C, it\'s legal to write something like:

int foo = +4;

However, as far as I can tell, the unary plus (+) in +4

Answer 1

There's one very handy use of the unary plus operator I know of: in macros. Suppose you want to do something like

#if FOO > 0

If FOO is undefined, the C language requires it be replaced by 0 in this case. But if FOO was defined with an empty definition, the above directive will result in an error. Instead you can use:

#if FOO+0 > 0

And now, the directive will be syntactically correct whether FOO is undefined, defined as blank, or defined as an integer value.

Of course whether this will yield the desired semantics is a completely separate question, but in some useful cases it will.

Edit: Note that you can even use this to distinguish the cases of FOO being defined as zero versus defined as blank, as in:

#if 2*FOO+1 == 1
/* FOO is 0 */
#else
/* FOO is blank */
#endif