Question

I have a test program like below.

  #define TEST(A,B)  A
  #define TEST2(A,B) (A,B)
  #define TEST3(A,B) TEST TEST2(A,B)

  int main()
  {
     TEST3(Hello,World)  //This will expand to TEST (Hello,World)
     TEST (hello, World) // This will expand to hello
  }

The TEST3 will expand to “TEST (Hello,World)“, but it won’t be expanded further using TEST definition. I initially thought it must be due to a space between TEST and TEST2(hello, world) in the TEST3 definition. But the plain invocation of TEST (hello, world) expands properly. Can someone explain what is happening here?

Answer 1

The word TEST in #define TEST3(A,B) TEST TEST2(A,B) is not a function-like macro invocation because it is not followed by an open parenthesis. When the pre-processor is expanding TEST3(Hello, World), it encounters TEST, finds it is not an invocation of a function-like macro, and outputs it as text; then it processes TEST2(A, B) and that is a macro invocation, so it outputs the corresponding text, which is (Hello,World), and it continues the processing with the open parenthesis. The TEST is gone, never to be preprocessed again.

See C preprocessor and concatenation for a full discussion of macro expansion with quotes from the standard. You might find How to make a char string from a C macro’s value of some help, too.