递归 C 宏未扩展 [英] Recursive C macro not expanded

问题描述

我正在研究一个递归宏.但是，它似乎没有递归扩展.这是一个最小的工作示例来说明我的意思:

I am working on a recursive macro. However, it seems that it is not expanded recursively. Here is a minimal working example to show what I mean:

// ignore input, do nothing
#define ignore(...)
// choose between 6 names, depending on arity
#define choose(_1,_2,_3,_4,_5,_6,NAME,...) NAME
// if more than one parameter is given to this macro, then execute f, otherwise ignore
#define ifMore(f,...) choose(__VA_ARGS__,f,f,f,f,f,ignore)(__VA_ARGS__)
// call recursively if there are more parameters
#define recursive(first,args...) first:ifMore(recursive,args)

recursive(a,b,c,d)
// should print: a:b:c:d
// prints: a:recursive(b,c,d)

recursive 宏应以递归方式扩展自身并始终连接结果，用冒号分隔.但是，它不起作用.递归宏被正确生成(从结果中可以看出 a:recursive(b,c,d) 再次包含对宏的格式良好的调用)，但生成的递归调用 ist没有展开.

The recursive macro should expand itself recursively and always concatenate the result, separated with a colon. However, it doesn't work. The recursive macro is generated correctly (as can be seen on the result a:recursive(b,c,d) which includes a well-formed call to the macro again), but the generated recursive call ist not exanded.

为什么会这样，我怎样才能得到我想要的行为?

Why is this the case and how can I get the behaviour I want?

推荐答案

你无法得到你想要的行为.根据设计，C 预处理器不是图灵完备的.

You can't get the behaviour you want. The C preprocessor is, by design, not turing complete.

您可以使用多个宏来获得多个替换，但使用任意数量的替换将无法实现真正​​的递归.

You can use multiple macros to get multiple replacements, but you will not achieve true recursion with an arbitrary number of replacements.

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