How do I implement no-op macro (or template) in C++? - c++

Question

How do I implement no-op macro in C++?
#include <iostream>
#ifdef NOOP
#define conditional_noop(x) what goes here?
#else
#define conditional_noop(x) std::cout << (x)
#endif
int main() {
conditional_noop(123);
}
I want this to do nothing when NOOP is defined and print "123", when NOOP is not defined.

Answer 1

While leaving it blank is the obvious option, I'd go with
#define conditional_noop(x) do {} while(0)
This trick is obviously no-op, but forces you to write a semicolon after conditional_noop(123).

Answer 2

As mentioned before - nothing.
Also, there is a misprint in your code.
it should be #else not #elif. if it is #elif it is to be followed by the new condition
#include <iostream>
#ifdef NOOP
#define conditional_noop(x) do {} while(0)
#else
#define conditional_noop(x) std::cout << (x)
#endif
Have fun coding!
EDIT: added the [do] construct for robustness as suggested in another answer.

Answer 3

Defining the macro to be void conveys your intent well.
#ifdef NOOP
#define conditional_noop(x) (void)0
#else

Answer 4

#ifdef NOOP
#define conditional_noop(x)
#elif
nothing!

Answer 5

#ifdef NOOP
static inline void conditional_noop(int x) { }
#else
static inline void conditional_noop(int x) { std::cout << x; }
#endif
Using inline function void enables type checking, even when NOOP isn't defined. So when NOOP isn't defined, you still won't be able to pass a struct to that function, or an undefined variable.
This will eventually prevent you from getting compiler errors when you turn the NOOP flag on.

Answer 6

You can just leave it blank. You don't need to follow the #define with anything.

Answer 7

Like others have said, leave it blank.
A trick you should use is to add (void)0 to the macro, forcing users to add a semicolon after it:
#ifdef NOOP
#define conditional_noop(x) (void)0
#else
#define conditional_noop(x) std::cout << (x); (void)0
#endif
In C++, (void)0 does nothing. This article explains other not-as-good options, as well as the rationale behind them.

Answer 8

As this is a macro, you should also consider a case like
if (other_cond)
conditional_noop(123);
to be on the safe side, you can give an empty statement like
#define conditional_noop(X) {}
for older C sometimes you need to define the empty statment this way (should also get optimized away):
#define conditional_noop(X) do {} while(0)

Answer 9

I think that a combination of the previous variants is a good solution:
#ifdef NOOP
static inline void conditional_noop(int x) do {} while(0)
#else
static inline void conditional_noop(int x) do { std::cout << x; } while(0)
#endif
The good thing is that these two codes differ only inside a block, which means that their behaviour for the outside is completely identical for the parser.