C++ 一行中的多个预处理器指令
[英]Multiple preprocessor directives on one line in C++
问题描述
A hypothetical question: Is it possible to have a C++ program, which includes preprocessor directives, entirely on one line?
一个假设的问题：是否有可能将包含预处理器指令的 C++ 程序完全放在一行中？
Such a line would look like this:这样的一行看起来像这样：
#define foo #ifdef foo #define bar #endif
What are the semantics of such a line?这条线的语义是什么？
Further, are there any combinations of directives which are impossible to construct on one line?此外，是否存在不可能在一行中构建的指令组合？
If this is compiler-specific then both VC++ and GCC answers are welcome.如果这是特定于编译器的，那么欢迎使用 VC++ 和 GCC 答案。
2 个解决方案
解决方案1
15 已采纳 2010-08-20 15:22:27
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:预处理指令必须以换行符结束，所以这实际上是一个单独的预处理指令，它定义了一个类对象宏，名为foo ，扩展为以下标记序列：
# ifdef foo # define bar # endif
Any later use of the name foo in the source (until it is #undef ed) will expand to this, but after the macro is expanded, the resulting tokens are not evaluated as a preprocessing directive.以后在源代码中使用名称foo （直到它被#undef编辑）将扩展为此，但是在扩展宏之后，生成的标记不会被评估为预处理指令。
This is not compiler-specific;这不是特定于编译器的； this behavior is defined by the C and C++ standards.此行为由 C 和 C++ 标准定义。
解决方案2
7 2010-08-20 15:36:17
Preprocessor directives are somewhat different than language statements, which are terminated by ;预处理器指令与语言语句有些不同，语言语句以;终止。 and use whitespace to delimit tokens.并使用空格来分隔标记。 In the case of the preprocessor, the directive is terminated by a newline so it's impossible to do what you're attempting using the C++ language itself.在预处理器的情况下，该指令以换行符终止，因此不可能使用 C++ 语言本身来执行您尝试执行的操作。
One way you could kind of simulate this is to put your desired lines into a separate header file and then #include it where you want.您可以模拟这种情况的一种方法是将所需的行放入单独的头文件中，然后将其#include到您想要的位置。 The separate header still has to have each directive on one line, but the point where you include it is just a single line, effectively doing what you asked.单独的标头仍然必须将每个指令放在一行中，但是您包含它的地方只是一行，可以有效地执行您的要求。
Another way to accomplish something like that is to have a pre-C++ file that you use an external process to process into a C++ source file prior to compiling with your C++ compiler.完成类似任务的另一种方法是在使用 C++ 编译器进行编译之前，使用外部进程将 C++ 前文件处理成 C++ 源文件。 This is probably rather more trouble than it's worth.这可能比它的价值更麻烦。
解决方案3
1 2020-06-16 13:37:57
A hypothetical question: Is it possible to have a C++ program, which includes preprocessor directives, entirely on one line?一个假设的问题：是否有可能在一行中包含一个包含预处理器指令的 C++ 程序？
Such a line would look like this:这样的一行看起来像这样：
#define foo #ifdef foo #define bar #endif
What are the semantics of such a line?这样一行的语义是什么？
Further, are there any combinations of directives which are impossible to construct on one line?此外，是否存在无法在一行中构建的指令组合？
If this is compiler-specific then both VC++ and GCC answers are welcome.如果这是特定于编译器的，那么欢迎 VC++ 和 GCC 答案。
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