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

Question

How do I implement no-op macro in C++?

#include    

#ifdef NOOP       
    #define conditional_noop(x) what goes here?   
#else       
    #         


        

Answer 1

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 2

 #ifdef NOOP       
     #define conditional_noop(x)   
 #elif  

nothing!

Answer 3

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)