使用偏移量获取宏值的子集 [英] Get subsets of macro values using offset

问题描述

我有一个列表定义为预处理器值#define LIST 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.我想编写一个获取索引0或1并计算为LIST子集的宏，这样对于索引0它将评估为0, 2, 4, 6, 8，对于索引1它将评估为1, 3, 5, 7, 9.可以保证LIST的长度是偶数，但我事先不知道内容(它是由我提供的库的用户自动生成的).此问题是对此问题

I have a list defined as a preprocessor value #define LIST 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. I want to write a macro that gets an index 0 or 1 and evaluates to a subset of the LIST such that for index 0 it will evaluate to 0, 2, 4, 6, 8 and for index 1 it will evaluate to 1, 3, 5, 7, 9. It is guaranteed that LIST's length is even but I don't know the content in advance (it is auto generated by the users of the library I supply). This question is a follow up on this question

#define LIST 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
#define MACRO(index) \
use LIST and index

// For LIST that given in the example
printf("%d %d %d %d %d\n", MACRO(0)); // print 0 2 4 6 8
printf("%d %d %d %d %d\n", MACRO(1)); // print 1 3 5 7 9

推荐答案

在 [实时示例]

通过将LIST转换为Boost.Preprocessor sequence ，然后对其进行迭代并仅保留索引模2与MACRO的index参数匹配的那些元素，最后进行工作将生成的序列转回以逗号分隔的列表.

It works by turning LIST into a Boost.Preprocessor sequence, then iterating over it and keeping only those elements whose index modulo 2 matches the index parameter of MACRO, and finally turning the resulting sequence back into a comma-separated list.

请注意，Boost.Preprocessor元组的最大大小是有限制的.在版本1.66.0(撰写本文时为最新版本)中，它是64.如果您的LIST较大，则不能将其视为元组(上面的代码通过使用(LIST)将一对括号括起来)它).

Note that there is a limit on the maximum size of a Boost.Preprocessor tuple. In version 1.66.0 (the latest as of this writing), it's 64. If your LIST is larger, it cannot be treated as a tuple (which the code above does by using (LIST) to put a pair of parentheses around it).

如果您可以控制LIST的格式，则可以直接将其更改为Boost.Preprocessor序列:

If you have control over the format of LIST, you could change it do be a Boost.Preprocessor sequence directly:

#define LIST (0)(1)(2)(3)(4)(5)(6)(7)(8)(9)

虽然序列也有大小限制，但它要大得多(Boost 1.66.0中为256).然后，宏将更改如下:

While sequences have a size limit too, it's far larger (256 in Boost 1.66.0). The macro would then change as follows:

#define MACRO(index) \
  BOOST_PP_SEQ_ENUM( \
    BOOST_PP_SEQ_FOR_EACH_I( \
      OUTPUT_CORRECT_OFFSET, \
      index, \
      LIST \
    ) \
  )

如果序列限制仍然不够，您将不得不考虑使用更强大的代码生成技术，例如本机C ++框架之外的独立宏处理器.

If the sequence limit is still not enough, you will have to consider a more powerful code generation technique, such as a stand-alone macro processor outside the native C++ framework.

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