c - 在#define 之前使用#undef

Question

在许多地方，我在定义同一个宏之前看到了 undefine 宏的用法。例如:

#undef FORMULA
#ifdef SOMETHING
#define FORMULA 1
#else 
#define FORMULA 2
#endif

#undefine FORMULA 使用了什么？ 我可能猜想它处理的是之前已经定义了宏的情况。但是新定义不是覆盖了旧定义吗？谢谢!

Answer 1

当前定义的宏名称不能用 不同的 定义重新定义(见下文)，因此 #undef 允许用不同的定义重新定义该宏名称。

这是相关的法律术语:

C 和 C++ 标准(相同的措辞):

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.

措辞略有不同，意思相同:

C Standard (N1256), §6.10.3/2:
An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless 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.

C++ Standard (N3337) §16.3/2
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.

两个标准中的措辞相同:

Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.

所以:

#define X(y)   (y+1)
#define X(z)   (z+1)   // ill-formed, not identical

恕我直言，由于预处理器宏的范围规则，使用 #undef 通常很危险。我宁愿从预处理器获得警告或错误，并提出不同的预处理器宏，而不是让某些翻译单元默默地接受错误的宏定义，从而将错误引入程序。考虑:

// header1.h
#undef  PORT_TO_WRITE_TO
#define PORT_TO_WRITE_TO    0x400

// header2.h
#undef  PORT_TO_WRITE_TO
#define PORT_TO_WRITE_TO    0x410

并且有一个翻译单元 #include 两个标题。没有警告，可能不是预期的结果。