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

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.