Why is this macro replaced as 20 instead 10? - macros

Question

1. #define NUM 10
2. #define FOO NUM
3. #undef NUM
4. #define NUM 20
5.
6. FOO
When I only run the preprocessor, the output file contains 20.
However, from what I understand, the preprocessor simply does text replacement. So this is what I think is happening (which is obviously wrong but idky):
NUM is defined as 10.
Therefore, in line 2, NUM is replaced as 10. So now we have "#define FOO 10".
NUM is undefined.
NUM is redefined and now is 20.
FOO is replaced according to line 2, which was before line 4's redefinition, and is 10.
So I think the output should be 10 instead of 20. Can anything explain where it went wrong?

Answer 1

The text replacement is done where the macro is used, not where you wrote the #define. At the point you use FOO, it replaces FOO with NUM and NUM is currently defined to be 20.

Answer 2

In the interests of collecting all the relevant specifications from the standards, I extracted this information from a comment thread, and added C++ section numbers, based on draft N4527 (the normative text is identical in the two standards). The standard(s) are absolutely clear on the subject.
#define preprocessor directives do not undergo macro replacement.
(C11 §6.10¶7; C++ §16[cpp] ¶6): The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.
After a macro is replaced with its replacement text, the new text is rescanned. Preprocessor tokens in the replacement are expanded as macros if there is an active macro definition for the token at that point in the program.
(C11 §6.10.3¶9; C++ §16.3[cpp.replace] ¶9) A preprocessing directive of the form
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent instance of the macro name to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive. The replacement list is then rescanned for more macro names as specified below.
A macro definition is active from the line following the #define until an #undef for the macro name, or the end of the file.
(C11 §6.10.3.5¶1; C++ §16.3.5[cpp.scope] ¶1) A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the preprocessing translation unit. Macro definitions have no significance after translation phase 4.
If we look at the program:
#define NUM 10
#define FOO NUM
#undef NUM
#define NUM 20
FOO
we see that the macro definition of NUM in line 1 lasts exactly to line 3. There is no replaceable text in those lines, so the definition is never used; consequently, the program is effectively the same as:
#define FOO NUM
#define NUM 20
FOO
In this program, at the third line, there is an active definition for FOO, with replacement list NUM, and for NUM, with replacement list 20. The FOO is replaced with its replacement list, making it NUM, and then that is once again scanned for macros, resulting in NUM being replaced with its replacement list 20. That replacement is again rescanned, but there are no defined macros, so the end result is that the token 20 is left for processing in translation phase 5.

Answer 3

In:
FOO
the preprocessor will replace it with NUM, then it will replace NUM with what it is currently defined as, which is 20.
Those initial four lines are equivalent to:
#define FOO NUM
#define NUM 20

Answer 4

The C11 standard says (and other versions of C, and C++, say similarly):
A preprocessing directive of the form # define identifier replacement-list new-line defines an object-like macro that causes each subsequent instance of the macro name to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive. The replacement list is then rescanned for more macro names as specified below.
However it also says in another part (thanks to rici for pointing this out).
The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.
So a subsequent instance of the macro name which is found inside another #define directive is actually not replaced.
Your line #define FOO NUM defines that when the token FOO is later found (outside of another #define directive!), it will be replaced by the token NUM .
After a token is replaced, rescanning occurs, and if NUM is itself a macro, then NUM is replaced at that point. (And if whatever NUM expands to contains macros , then that gets expanded , and so on).
So your sequence of steps is actually:
NUM defined as 10
FOO defined as NUM
NUM undefined and re-defined as 20
FOO expands to NUM
(rescan) NUM expands to 20
This behaviour can be seen in another common preprocessor trick, to turn the defined value of a macro into a string:
#define STR(X) #X
#define STR_MACRO(X) STR(X)
#define NUM 10
puts( STR_MACRO(NUM) ); // output: 10
If we had written puts( STR(NUM) ) then the output would be NUM.
The output of 10 is possible because, as before, the second #define here does not actually expand out STR. So the sequence of steps in this code is:
STR(X) defined as #X
STR_MACRO(X) defined as STR(X)
NUM defined as 10
STR_MACRO and NUM are both expanded; the result is puts( STR(10) );
(Rescan result of last expansion) STR(10) is expanded to "10"
(Rescan result of last expansion) No further expansion possible.