c++ - What is the # for when formatting using %s

Question

I came across this example of an assertion and was wondering what the # is for:

#define ASSERT( x ) if ( !( x ) ) { 
    int *p = NULL; 
    DBGPRINTF("Assert failed: [%s]
 Halting.", #x); 
    *p=1; 
  } 

Answer 1

It is the "stringize" preprocessing operator.

It takes the tokens passed as the argument to the macro parameter x and turns them into a string literal.

#define ASSERT(x) #x

ASSERT(a b c d)
// is replaced by
"a b c d"