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]\r\n 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"

Answer 2

#x is the stringification directive

#define Stringify(x) #x

means Stringify(abc) will be substituted with "abc"

as in

#define initVarWithItsOwnName(x) const char* p = #x

int main()
{
   initVarWithItsOwnName(Var);
   std::cout << Var; //will print Var
}