Is it possible to define another preprocessor directive?

Question

I've been looking through code golf and got an idea to try this code:

#define D #define after adding this line, everything worked fine, however I expanded it into this:

#define D #define
D VALUE

And here I got 5 compilation error. If I change D into #define everything is fine, can someone explain, why this code is illegal?

NOTE: I used VS2008 compiler.

EDIT: After some answers I see that I needed to give compilations error list:

1. error C2121: '#' : invalid character : possibly the result of a macro expansion
2. error C2146: syntax error : missing ';' before identifier 'VALUE'
3. error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
4. error C2144: syntax error : 'void' should be preceded by ';'
5. error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

First error shows that D is not just define but also includes #.

Answer 1

C 2011 (N1570) 6.10.3.4 3: “The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one,…”

C++ 2010 (N3092) 16.3.4 [cpp.rescan] 3 has exactly the same text.

Answer 2

This code is illegal because language specification says it is illegal. According to C and C++ preprocessor specification, whatever code you build using preprocessor will never be interpreted as another preprocessor directive. In short, you cannot build preprocessor directives using preprocessor. Period.

(Also, you cannot build comments using preprocessor.)

Answer 3

It does look like your preprocessor is making the substitution you want, but you likely wouldn't get the behaviour you want - the preprocessor is normally just a single pass operation. Example (with clang, but you should be able to reproduce by using the appropriate VS2008 flags):

$ cat example.c 
#define D #define
D VALUE
$ cc -P -E example.c 

 #define VALUE

That #define VALUE is going straight through to the compiler, which won't know what to do with it - it's a preprocessor directive, after all. Clang's error, for reference, is similar to yours:

$ cc -c example.c 
example.c:2:1: error: expected identifier or '('
D VALUE
^
example.c:1:11: note: expanded from macro 'D'
#define D #define
          ^
1 error generated.