Passing parameters to a no-op macro in C++

Question

I am getting the following error message

error: '0' cannot be used as a function

when trying to compile the following line:

NOOP(0 != width);

NOOP is defined as follows:

#define NOOP (void)0

The source code is part of a SDK - so it should be okay. And I have found out that (void)0 actually is a valid way to descibe "no operation" in C++. But why would you want to pass a boolean parameter to a function which does nothing? And how do you get rid of the error message?

Answer 1

The MACRO is not defined with any parameters on it, so after the preprocessor replaces code, that statement ends up looking like this:

(void)0(0 != width);

Which confuses the compiler into thinking you are trying to use the "()" operator on 0. (ie using 0 as a function)

I recommend that you drop the "(0 != width)" (it is misleading) and just write NOOP;

Answer 2

" (void)0(0!=width); " is not valid C++, so it's not OK. (void)0; by itself doesn't do anything in C++, so can be used as a noop. Instead of your current define, I would use:

#define NOOP(X) (void)0

This tells the C++ preprocessor that there is a preprocessor function called NOOP that takes one parameter of any type, and replaces that entire function call with (void)0 . So if you have a line of code that says NOOP("HELLO WORLD") , then the preprocessor replaces that entire thing with (void)0 , which the C++ compiler proceeds to ignore.