如何使用Boost预处理程序多次打印逗号 [英] How do I print out a comma multiple times using Boost Preprocessor

问题描述

我需要使用可变宏来扩展到类的多个变体。因为他们需要根据宏输入具有不同的名称，我不能简单地使用模板。问题是，我不能扩展逗号（，）符号，我的类有多个参数（我需要使用逗号符号）的函数。

I need to use a variadic macro to expand to multiple variations of a class. Since they need to have different names based on the macro input I can't simply use templates. The problem is that I can't expand the comma (,) symbol, and my class has functions which take multiple parameters (for which I need to use the comma symbol).

boost 提供了 BOOST_PP_COMMA（）宏到逗号，但它只在循环结构外工作。我猜这个问题是 BOOST_PP_COMMA（）被展开一次，然后被当作逗号，在这一点程序中断。

boost provides the BOOST_PP_COMMA() macro which expands to a comma, but it only works outside of loop constructs. I'm guessing the issue is that BOOST_PP_COMMA() is expanded once and then treated as a comma, at which point the program breaks.

为了说明这个问题，假设我有一个宏函数，它接受一个可变参数，并产生一定数量的逗号等于赋给它的参数数量。天真的解决方案是：

To illustrate the problem, suppose I have a macro function which takes a variadic number of parameters and produces a number of commas equal to the number of parameters given to it. The naive solution would be this:

#define TEST(...)\
    BOOST_PP_REPEAT( \
        BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), \
        MACRO, \
        BOOST_PP_VARIADIC_TO_TUPLE(__VA_ARGS__))

#define MACRO(z, n, data) BOOST_PP_IF(1,BOOST_PP_COMMA(),BOOST_PP_COMMA())\

但这会产生一系列错误，

But this produces a range of errors because the comma is expanded and the macro thinks they're dividing parameters.

有什么办法解决这个问题吗？

Is there any way around this problem?

推荐答案

使用 BOOST_PP_REPEAT 与可以使用预期参数调用的宏可以正常工作，甚至可以防止 BOOST_PP_COMMA ：

Using BOOST_PP_REPEAT with a macro that can be called with the expected arguments will work fine, and it even prevents the need for BOOST_PP_COMMA:

#define PRINT_COMMAS(...)\
    BOOST_PP_REPEAT( \
        BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), \
        PRINT_COMMAS_MACRO, \
        BOOST_PP_VARIADIC_TO_TUPLE(__VA_ARGS__))

#define PRINT_COMMAS_MACRO(z, n, data) ,

查看工作

要保存额外的宏，您可以利用 BOOST_PP_ENUM 在扩展之间添加逗号，方法是使用 BOOST_PP_TUPLE_EAT ：将$ 1添加到重复数中并丢弃宏参数：

To save the extra macro, you can take advantage of the fact that BOOST_PP_ENUM adds commas between expansions by adding one to the number of repetitions and discarding the macro arguments using BOOST_PP_TUPLE_EAT:

#define PRINT_COMMAS(...)\
    BOOST_PP_ENUM( \
        BOOST_PP_INC(BOOST_PP_VARIADIC_SIZE(__VA_ARGS__)), \
        BOOST_PP_TUPLE_EAT(), \
        BOOST_PP_VARIADIC_TO_TUPLE(__VA_ARGS__))

查看工作

我个人认为第一个更清楚。

I personally think the first is more clear.

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