问题描述
一个假设的问题：是否可能有一个C ++程序，包括预处理指令，完全在一行上？
A hypothetical question: Is it possible to have a C++ program, which includes preprocessor directives, entirely on one line?
这样的行看起来像这样：
Such a line would look like this:
#define foo #ifdef foo #define bar #endif
这样的行的语义是什么？
What are the semantics of such a line?
此外，是否存在无法构造的指令组合？
Further, are there any combinations of directives which are impossible to construct on one line?
如果这是特定于编译器的，则欢迎使用VC ++和GCC答案。
If this is compiler-specific then both VC++ and GCC answers are welcome.
推荐答案
预处理指令必须以换行符终止，因此实际上这是一个单一的预处理指令，该指令定义了一个类似于对象的宏，名为 foo ，将扩展为以下标记序列：
A preprocessing directive must be terminated by a newline, so this is actually a single preprocessing directive that defines an object-like macro, named foo, that expands to the following token sequence:
# ifdef foo # define bar # endif
以后在源中使用名称 foo 的任何用法（直到 #undef ed）将扩展为此，但
Any later use of the name foo in the source (until it is #undefed) will expand to this, but after the macro is expanded, the resulting tokens are not evaluated as a preprocessing directive.
这不是特定于编译器的；而是将宏扩展后，才将它们作为预处理指令进行评估。这种行为由C和C ++标准定义。
This is not compiler-specific; this behavior is defined by the C and C++ standards.
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