使用C预处理程序的嵌套宏迭代 [英] Nested macro iteration with C preprocessor

问题描述

使用C预处理器，您可以具有某种高阶宏.像这样:

With the C preprocessor you can have some kind of high-order macros. Something like this:

#define ABC(f) f(a) f(b) f(c)
#define XY(f) f(x) f(y)

#define CODE(x) foo_ ## x
ABC(CODE)
#undef CODE

#define CODE(x) bar_ ## x
XY(CODE)
#undef CODE

输出为:

 foo_a foo_b foo_c
 bar_x bar_y

是否有一些技巧可以嵌套这样的迭代，以进行类似的事情?

Is there some trick to nest such iterations, to do something like this?

#define CODE(x) foo_ ## x
NEST(ABC, XY, CODE)
#undef CODE

所以输出将是:

foo_ax foo_ay foo_bx foo_by foo_cx foo_cy

尤其是，我希望ABC和XY的定义彼此独立，以便我仍然可以独立使用ABC甚至可以做这样的事情:

In particular, I'd like to have the definitions of ABC and XY independent from each other, so that I can still use ABC stand-alone or maybe do even something like this:

#define CODE(x) foo_ ## x
NEST(XY, KLMN, ABC, CODE)
#undef CODE

---

记录下来，这里是解决方案:

---

For the record, here the solution:

#include <boost/preprocessor/seq.hpp>

#define ABC (a) (b) (c)
#define XY (x) (y)

#define CODE(r, prod) BOOST_PP_CAT(foo_, BOOST_PP_SEQ_CAT(prod))
BOOST_PP_SEQ_FOR_EACH_PRODUCT(CODE, (ABC) (XY))

收益:

foo_ax foo_ay foo_bx foo_by foo_cx foo_cy

推荐答案

增强预处理器库提供了一些可以立即使用的宏.

The Boost Preprocessor Library offers several macros that can do this out of the box.

BOOST_PP_SEQ_FOR_EACH_PRODUCT 将进行迭代在两个或多个以(x) (y) (z)形式编码的列表的笛卡尔积上(称为序列是库的说法).

BOOST_PP_SEQ_FOR_EACH_PRODUCT will iterate over the cartesian products of two or more lists encoded in the form (x) (y) (z) (which are known as sequences is the library's parlance).

BOOST_PP_LIST_FOR_EACH_PRODUCT 可以编码为(x, (y, (z, NIL)))的列表也是如此.

BOOST_PP_LIST_FOR_EACH_PRODUCT will do the same for lists encoded as (x, (y, (z, NIL))).

将 X宏迭代转换为这样的序列"很简单: /p>

It's trivial to convert the X macro iteration to a "sequence" like this:

#define LIST_TO_SEQ(X) (X)
ABC(LIST_TO_SEQ)

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