I am trying to create a "single line comment" macro in **C**, this is used conditionally to comment out lines of codes, according to some global macro definitions. It is the same idea expressed in [this article](http://www.drdobbs.com/cc-tip-5-a-cc-comment-macro/184401344).

Trying many permutations of this code, I keep getting error messages from the compiler.

For example, directly following the code sample from that page:

```
#define COMMENT SLASH(/)
#define SLASH(s) /##s
#define DEBUG_ONLY COMMENT
DEBUG_ONLY a = b;   // <-- line #83

```

GCC gives the following error:

> 
> prog.c:83:1: error: pasting "/" and "/" does not give a valid preprocessing token  
> 
>  prog.c:83: error: expected expression before ‘/’ token
> 
> 
> 

As mentioned, I played with that theme and tried many variations but all fail giving similar diagnostics.

What am I doing wrong, and why doesn't the code from the article compiles well?