C和C ++中的宏重新定义

Question

I know that this code is valid both in C and C++:

#define FOO 0
#define FOO 0

ISO/IEC 14882:2011

16.3 Macro replacement [cpp.replace]

2 An identifier currently defined as an object-like macro may be redefined by another #define preprocessing directive provided that the second definition is an object-like macro definition and the two replacement lists are identical, otherwise the program is ill-formed. Likewise, an identifier currently defined as a function-like macro may be redefined by another #define preprocessing directive provided that the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical, otherwise the program is ill-formed.

But what about this code?

#define FOO 0
#define FOO FOO

Replacement lists are not identical at the start of preprocessing (only when the first replacement occurs).

Answer 1

This is not allowed in either C or C++. The replacement list must be identical. What you're talking about (after the first pass) is the result of processing the replacement list ¹ , not the replacement list itself. Since the replacement list itself is not identical, the code is not allowed.

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¹ Or at least what the result would be if the preprocessor worked a particular way that happens to be different from how it actually does.