如何评估嵌套的预处理器宏 [英] How to evaluate a nested preprocessor macro

问题描述

假设我想选择某个预处理器指令的行为，该指令在编译时评估一个常量字符串的连接和另一个宏的结果.

let's say I want to select the behaviour of a certain preprocessor directive evaluating at compile time the concatenation of a constant string and the result of another macro.

#define CASE1 text1
#define CASE2 text2
#define CASE3 text3
#define SCENARIO 3
/** the following won't work - for examplification purposes only**/
#define FUNCTION CASE##SCENARIO

/** whenever I write FUNCTION, I expect to see text3 **/

我很难想出一个可行的解决方案，因为预处理器是一次性的野兽.这甚至可行吗?

I am having an hard time thinking of a viable solution, as the preprocessor is a one-pass beast. Is that even feasible ?

推荐答案

有可能，你只需要添加一些额外的宏层.关键是当你使用标记粘贴操作符##时，预处理器不会扩展它的操作数.但是，如果您添加另一层宏，预处理器将扩展这些参数.例如:

It's possible, you just need to add some extra layers of macros. The key is that when you use the token-pasting operator ##, the preprocessor will not expand its operands. But, if you add another layer of macros, the preprocessor will expand those arguments. For example:

#define CASE1 text1
#define CASE2 text2
#define CASE3 text3
#define SCENARIO 3

#define TOKENPASTE_HELPER(x, y) x ## y
#define TOKENPASTE(x, y) TOKENPASTE_HELPER(x, y)
#define FUNCTION TOKENPASTE(CASE, SCENARIO)

当预处理器扩展 FUNCTION 时，它会扩展 TOKENPASTE.当它扩展 TOKENPASTE 时，它扩展了它的参数(所以 SCENARIO 被 3 替换)，因为它的参数都不是标记的操作数——粘贴运算符.接下来，它展开 TOKENPASTE_HELPER，它执行实际的令牌粘贴以生成 CASE3.最后，扩展CASE3宏得到text3.

When the preprocessor expands FUNCTION, it expands TOKENPASTE. When it expands TOKENPASTE, it expands its arugments (so SCENARIO gets replaced by 3), since neither of its arguments are operands of the token-pasting operator. Next, it expands TOKENPASTE_HELPER, which does the actual token pasting to make CASE3. Finally, it expands the CASE3 macro to get text3.

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