你的基本FOR_EACH_R
是正确的,导致问题的原因是调用func
在你的FOR_EACH_HELPER_R
macro.
您可以通过暂时删除它来验证这一点:
/* ... */
#define FOR_EACH_HELPER_R(func, sub, ...) (__VA_OPT__(FOR_EACH_AGAIN_R PARENS (func, __VA_ARGS__)), sub)
/* ... */
NEST_RECURSIVE(A, B, C)
神箭示例
会导致:
(((, C), B), A)
1. 宏观评估如何运作
宏观评估不是很直观,所以我将快速解释对此答案很重要的两个概念:
1.1 类函数宏求值顺序
Example:
#define FOO() 1
#define BAR() 2
#define BAZ(A, B) MIAU(A, B)
#define MIAU(A, B) B A
BAZ(FOO(), BAR())
当预处理器看到调用时BAZ()
将会发生以下事情:
-
- 参数经过充分评估:
-
- 计算值被代入宏体中:
-
- 宏观体全面评估again
因此,参数可能会被评估两次 - 一次是在它们被替换到宏主体之前,然后是在评估主体时再次。
这也是为什么您可以将宏作为参数传递给其他宏,例如:
#define FOO(fn) fn()
#define BAR() 12
FOO(BAR)
这里会发生以下事情:
-
- 预处理器完全评估参数:
BAR
确实命名了宏,但它不是宏调用。因此预处理器不会评估它并将其视为文本:BAR
-
- 代入宏体:
FOO(BAR)
-> BAR()
-
- 身材评价:
BAR()
-> 12
Your EXPAND
宏还使用它来重复强制表达式的求值。
1.2 不能从自身调用宏
e.g.:
#define FOO 1 + FOO
FOO
#define BAR 1 + BAZ
#define BAZ BAR
BAR
-
FOO
会导致1 + FOO
-
BAR
会导致1 + BAR
本质上,当预处理器正在评估给定的宏时,例如BAR
,任何发生BAR
在其中(或在它调用的宏之一中)将被标记为不要试图评估这一点 - 永远不要.
所以基本上一旦宏看到它自己的名字,游戏就结束了。
2. 为什么你的例子不起作用
让我们来看看您的评价FOR_EACH_R
宏:
(我将省略EXPAND
为简单起见,使用宏)
- First
EXPAND
round
- 首先我们从
FOR_EACH_HELPER_R(MY_FUNC, A, B, C)
- 替换到正文中:
MY_FUNC(FOR_EACH_AGAIN_R PARENS (MY_FUNC, B, C), A)
- then evaluate the body
- the preprocessor sees the call to
MY_FUNC
, so both arguments will be evaluated:
-
FOR_EACH_AGAIN_R PARENS (MY_FUNC, B, C)
变成FOR_EACH_AGAIN_R () (MY_FUNC, B, C)
(由于FOR_EACH_AGAIN_R
不是直接调用)
-
A
-> A
- 代入
MY_FUNC
body:
MY_FUNC((FOR_EACH_AGAIN_R () (MY_FUNC, B, C)), A)
->
(FOR_EACH_AGAIN_R () (MY_FUNC, B, C) | A)
- 评价身体:
(FOR_EACH_AGAIN_R () (MY_FUNC, B, C) | A)
->
(FOR_EACH_HELPER_R (MY_FUNC, B, C) | A)
->
预处理器检测递归(我们在MY_FUNC
被称为FOR_EACH_HELPER_R
我们正在尝试打电话FOR_EACH_HELPER_R
又在这里)
so the FOR_EACH_HELPER_R
将被标记为不要尝试评估这个
另外MY_FUNC
参数也将被标记(因为我们在MY_FUNC
)
- Every
EXPAND
在那之后
- 预处理器尝试计算当前表达式:
(FOR_EACH_HELPER_R (MY_FUNC, B, C) | A)
but FOR_EACH_HELPER_R
被标记为不要尝试评估这个,因此该调用被忽略并且没有任何内容被替换。
-> 你最终会得到(FOR_EACH_HELPER_R (MY_FUNC, B, C) | A)
作为输出
3. 如何修复
最大的问题是你通过了FOR_EACH_AGAIN_R(...)
作为你的论点func
,它将对该部分求值两次(一次作为参数,一次在主体中),因此预处理器会看到递归调用并停止。
您可以通过延迟来部分修复它FOR_EACH_AGAIN_R
通过另一个评估周期,例如:
/* ... */
#define FOR_EACH_HELPER_R(func, sub, ...) func (__VA_OPT__(FOR_EACH_AGAIN_R PARENS (func, __VA_ARGS__)), sub)
#define FOR_EACH_AGAIN_R() FOR_EACH_AGAIN_R_IMPL PARENS
#define FOR_EACH_AGAIN_R_IMPL() FOR_EACH_HELPER_R
/* ... */
神箭示例
这将导致:
(MY_FUNC (MY_FUNC (, C), B) | A)
现在循环已完全扩展,但是仍然存在递归问题MY_FUNC
.
这里的根本问题是参数之一MY_FUNC
将包含MY_FUNC
, e.g.:
MY_FUNC((FOR_EACH_AGAIN_R PARENS (MY_FUNC, B, C)), A)
所以一旦预处理器替换MY_FUNC
into MY_FUNC
该令牌将立即标记为永远不要再尝试评估它.
So the MY_FUNC
第一次调用后链卡住了。
如果您不需要递归调用,那么会容易得多,例如:
/* ... */
#define FOR_EACH_HELPER_R(func, sub, ...) __VA_OPT__(FOR_EACH_AGAIN_R PARENS (func, __VA_ARGS__)), func(sub)
/* ... */
#define MY_FUNC(var) (var)
/* ... */
神箭示例
会毫无问题地工作(结果是, (C), (B), (A)
)
如果您绝对需要递归调用,那么只有一种方法:
你需要确保MY_FUNC
永远看不到FOR_EACH_HELPER_R
& MY_FUNC
.
但考虑到每次调用MY_FUNC
需要先前调用的结果,您唯一的选择是以所有MY_FUNC
呼叫立即进行评估。
例如你需要建立FOR_EACH_HELPER_R
以这样的方式,最终你会留下:
MY_FUNC(MY_FUNC(MY_FUNC(, C), B), A)
以便正确评估递归调用。
确保这一点的最简单方法是使用与您使用的相同的延迟技巧FOR_EACH_AGAIN_R
,例如有一组像这样的宏:
#define DELAY6(...) DELAY6_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY5(...) DELAY5_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY4(...) DELAY4_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY3(...) DELAY3_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY2(...) DELAY2_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY1(...) DELAY1_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY0(...) __VA_ARGS__
#define DELAY6_IMPL() DELAY5
#define DELAY5_IMPL() DELAY4
#define DELAY4_IMPL() DELAY3
#define DELAY3_IMPL() DELAY2
#define DELAY2_IMPL() DELAY1
#define DELAY1_IMPL() DELAY0
So DELAY6
将延迟6次评估,DELAY5
5个等...
然后你可以用它来延迟调用MY_FUNC
, e.g.:
#define FOR_EACH_R(func, ...) __VA_OPT__(EXPAND(FOR_EACH_HELPER_R(func, DELAY6, __VA_ARGS__)))
#define FOR_EACH_HELPER_R(func, del, sub, ...) del(func) (__VA_OPT__(FOR_EACH_AGAIN_R PARENS (func, del(), __VA_ARGS__)), sub)
注意我们正在经过del()
, not del
到下一个迭代FOR_EACH_HELPER_R
,这将有效地导致下一个较低的DELAY*
正在传递的函数(以便所有延迟都在一次评估中解决)
With NEST_RECURSIVE(A, B, C, D, E, F, G)
这将评估如下:
DELAY5_IMPL () (MY_FUNC) (DELAY5_IMPL () (MY_FUNC) (FOR_EACH_AGAIN_R () (MY_FUNC, DELAY5_IMPL () , C, D, E, F, G), B), A)
->
DELAY4_IMPL () (MY_FUNC) (DELAY4_IMPL () (MY_FUNC) (DELAY4_IMPL () (MY_FUNC) (FOR_EACH_AGAIN_R () (MY_FUNC, DELAY4_IMPL () , D, E, F, G), C), B), A)
->
DELAY3_IMPL () (MY_FUNC) (DELAY3_IMPL () (MY_FUNC) (DELAY3_IMPL () (MY_FUNC) (DELAY3_IMPL () (MY_FUNC) (FOR_EACH_AGAIN_R () (MY_FUNC, DELAY3_IMPL () , E, F, G), D), C), B), A)
->
DELAY2_IMPL () (MY_FUNC) (DELAY2_IMPL () (MY_FUNC) (DELAY2_IMPL () (MY_FUNC) (DELAY2_IMPL () (MY_FUNC) (DELAY2_IMPL () (MY_FUNC) (FOR_EACH_AGAIN_R () (MY_FUNC, DELAY2_IMPL () , F, G), E), D), C), B), A)
->
DELAY1_IMPL () (MY_FUNC) (DELAY1_IMPL () (MY_FUNC) (DELAY1_IMPL () (MY_FUNC) (DELAY1_IMPL () (MY_FUNC) (DELAY1_IMPL () (MY_FUNC) (DELAY1_IMPL () (MY_FUNC) (FOR_EACH_AGAIN_R () (MY_FUNC, DELAY1_IMPL () , G), F), E), D), C), B), A)
->
((((((( | G) | F) | E) | D) | C) | B) | A)
神箭示例
请注意MY_FUNC
直到最后一轮评估才被调用 - 这本质上确保了所有 MY_FUNC 调用都会立即评估,并且我们不会遇到递归宏调用的任何问题。
你必须定义很多DELAY_
不过要使其起作用(每个附加参数还有 1 个延迟宏)FOR_EACH_R
)
完整代码示例:godbolt
#define PARENS ()
#define EXPAND(...) EXPAND4(EXPAND4(EXPAND4(EXPAND4(__VA_ARGS__))))
#define EXPAND4(...) EXPAND3(EXPAND3(EXPAND3(EXPAND3(__VA_ARGS__))))
#define EXPAND3(...) EXPAND2(EXPAND2(EXPAND2(EXPAND2(__VA_ARGS__))))
#define EXPAND2(...) EXPAND1(EXPAND1(EXPAND1(EXPAND1(__VA_ARGS__))))
#define EXPAND1(...) __VA_ARGS__
#define DELAY6(...) DELAY6_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY5(...) DELAY5_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY4(...) DELAY4_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY3(...) DELAY3_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY2(...) DELAY2_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY1(...) DELAY1_IMPL PARENS __VA_OPT__((__VA_ARGS__))
#define DELAY0(...) __VA_ARGS__
#define DELAY6_IMPL() DELAY5
#define DELAY5_IMPL() DELAY4
#define DELAY4_IMPL() DELAY3
#define DELAY3_IMPL() DELAY2
#define DELAY2_IMPL() DELAY1
#define DELAY1_IMPL() DELAY0
#define FOR_EACH_R(func, ...) __VA_OPT__(EXPAND(FOR_EACH_HELPER_R(func, DELAY6, __VA_ARGS__)))
#define FOR_EACH_HELPER_R(func, del, sub, ...) del(func) (__VA_OPT__(FOR_EACH_AGAIN_R PARENS (func, del(), __VA_ARGS__)), sub)
#define FOR_EACH_AGAIN_R() FOR_EACH_HELPER_R
#define MY_FUNC(nested, var) (nested | var)
#define NEST_RECURSIVE(...) FOR_EACH_R(MY_FUNC, __VA_ARGS__)
NEST_RECURSIVE(A, B, C, D, E, F, G)
4. 可能更好的方法
4.1 简单宏链
上述解决方案需要很好地理解如何评估宏以了解发生的情况。您还可以通过定义一堆宏来选择更简单的方法,例如例如,升压确实
e.g.:
#define FOR_EACH_ERROR()
#define FOR_EACH_R(fn, ...) __VA_OPT__(FOR_EACH_R_IMPL_0(fn, __VA_ARGS__))
#define FOR_EACH_R_IMPL_0(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_1(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_1(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_2(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_2(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_3(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_3(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_4(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_4(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_5(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_5(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_6(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_6(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_7(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_7(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_8(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_8(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_9(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_9(fn, el, ...) fn(__VA_OPT__(FOR_EACH_R_IMPL_10(fn, __VA_ARGS__)), el)
#define FOR_EACH_R_IMPL_10(...) FOR_EACH_ERROR("Shenanigans!")
#define MY_FUNC(nested, var) (nested | var)
#define NEST_RECURSIVE(...) FOR_EACH_R(MY_FUNC, __VA_ARGS__)
NEST_RECURSIVE(A, B, C, D, E, F, G)
神箭示例
这更容易理解并且也很容易扩展(只需添加更多宏)
4.2 递归左折叠
如果您希望递归版本减少需要编写的行数,可以通过使用左折叠来实现,例如:
// Recursive Left Fold
#define FOR_EACH_L(fn, ...) __VA_OPT__(FOR_EACH_APPLY0(FOR_EACH_RESULT, FOR_EACH_L_4(fn,,__VA_ARGS__)))
#define FOR_EACH_L_4(fn, res, ...) FOR_EACH_APPLY4(FOR_EACH_L_3, FOR_EACH_APPLY4(FOR_EACH_L_3, FOR_EACH_APPLY4(FOR_EACH_L_3, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_3(fn, res, ...) FOR_EACH_APPLY3(FOR_EACH_L_2, FOR_EACH_APPLY3(FOR_EACH_L_2, FOR_EACH_APPLY3(FOR_EACH_L_2, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_2(fn, res, ...) FOR_EACH_APPLY2(FOR_EACH_L_1, FOR_EACH_APPLY2(FOR_EACH_L_1, FOR_EACH_APPLY2(FOR_EACH_L_1, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_1(fn, res, ...) FOR_EACH_APPLY1(FOR_EACH_L_0, FOR_EACH_APPLY1(FOR_EACH_L_0, FOR_EACH_APPLY1(FOR_EACH_L_0, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_0(fn, res, ...) fn, FOR_EACH_FIRST(__VA_OPT__(fn(res, FOR_EACH_FIRST(__VA_ARGS__)), ) res) __VA_OPT__(FOR_EACH_TAIL(__VA_ARGS__))
#define FOR_EACH_APPLY4(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY3(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY2(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY1(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY0(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_FIRST(el, ...) el
#define FOR_EACH_TAIL(el, ...) __VA_OPT__(, __VA_ARGS__)
#define FOR_EACH_RESULT(fn, res, ...) res
神箭示例
左折叠比右折叠更容易实现,因为获取第一个元素__VA_ARGS__
比获得最后一个容易得多(FOR_EACH_FIRST
在上面的例子中)。
上面提供的版本最多可以处理 81 个参数,如果您需要更多,只需创建更多版本FOR_EACH_L_*
& FOR_EACH_APPLY*
宏。 (这些宏中的每个附加宏都会使其可以处理的最大参数数量增加三倍)
不过,您确实需要以相反的顺序提供参数,因为它是左折叠,例如:
NEST_RECURSIVE(A, B, C, D, E, F, G)
// would result in ((((((( | A) | B) | C) | D) | E) | F) | G)
如果您需要右折叠,您可以通过反转参数然后调用我们上面创建的左折叠变体来实现它。
e.g.:
// Reverse args
#define PARENS ()
#define EXPAND(...) EXPAND4(EXPAND4(EXPAND4(EXPAND4(__VA_ARGS__))))
#define EXPAND4(...) EXPAND3(EXPAND3(EXPAND3(EXPAND3(__VA_ARGS__))))
#define EXPAND3(...) EXPAND2(EXPAND2(EXPAND2(EXPAND2(__VA_ARGS__))))
#define EXPAND2(...) EXPAND1(EXPAND1(EXPAND1(EXPAND1(__VA_ARGS__))))
#define EXPAND1(...) __VA_ARGS__
#define REVERSE(...) __VA_OPT__(EXPAND(REVERSE_HELPER(__VA_ARGS__)))
#define REVERSE_HELPER(el, ...) __VA_OPT__(REVERSE_AGAIN PARENS (__VA_ARGS__), ) el
#define REVERSE_AGAIN() REVERSE_HELPER
神箭示例
(这也仅适用于最多 81 个参数,您可以添加更多EXPAND*
宏来增加它可以处理的参数数量)
example:
REVERSE(A, B, C, D, E, F, G)
// would result in G, F, E, D, C, B, A
然后你可以像这样实现正确的折叠:
// Right fold
#define FOR_EACH_R(fn, ...) __VA_OPT__(FOR_EACH_R_APPLY(FOR_EACH_L, fn, REVERSE(__VA_ARGS__)))
#define FOR_EACH_R_APPLY(fn, ...) fn(__VA_ARGS__)
神箭示例
最终给出预期结果(最多 81 个参数),例如:
NEST_RECURSIVE(A, B, C, D, E, F, G)
// would result in: ((((((( | G) | F) | E) | D) | C) | B) | A)
完整代码:godbolt
// Reverse args
#define PARENS ()
#define EXPAND(...) EXPAND4(EXPAND4(EXPAND4(EXPAND4(__VA_ARGS__))))
#define EXPAND4(...) EXPAND3(EXPAND3(EXPAND3(EXPAND3(__VA_ARGS__))))
#define EXPAND3(...) EXPAND2(EXPAND2(EXPAND2(EXPAND2(__VA_ARGS__))))
#define EXPAND2(...) EXPAND1(EXPAND1(EXPAND1(EXPAND1(__VA_ARGS__))))
#define EXPAND1(...) __VA_ARGS__
#define REVERSE(...) __VA_OPT__(EXPAND(REVERSE_HELPER(__VA_ARGS__)))
#define REVERSE_HELPER(el, ...) __VA_OPT__(REVERSE_AGAIN PARENS (__VA_ARGS__), ) el
#define REVERSE_AGAIN() REVERSE_HELPER
// Recursive Left Fold
#define FOR_EACH_L(fn, ...) __VA_OPT__(FOR_EACH_APPLY0(FOR_EACH_RESULT, FOR_EACH_L_4(fn,,__VA_ARGS__)))
#define FOR_EACH_L_4(fn, res, ...) FOR_EACH_APPLY4(FOR_EACH_L_3, FOR_EACH_APPLY4(FOR_EACH_L_3, FOR_EACH_APPLY4(FOR_EACH_L_3, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_3(fn, res, ...) FOR_EACH_APPLY3(FOR_EACH_L_2, FOR_EACH_APPLY3(FOR_EACH_L_2, FOR_EACH_APPLY3(FOR_EACH_L_2, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_2(fn, res, ...) FOR_EACH_APPLY2(FOR_EACH_L_1, FOR_EACH_APPLY2(FOR_EACH_L_1, FOR_EACH_APPLY2(FOR_EACH_L_1, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_1(fn, res, ...) FOR_EACH_APPLY1(FOR_EACH_L_0, FOR_EACH_APPLY1(FOR_EACH_L_0, FOR_EACH_APPLY1(FOR_EACH_L_0, fn, res __VA_OPT__(, __VA_ARGS__))))
#define FOR_EACH_L_0(fn, res, ...) fn, FOR_EACH_FIRST(__VA_OPT__(fn(res, FOR_EACH_FIRST(__VA_ARGS__)), ) res) __VA_OPT__(FOR_EACH_TAIL(__VA_ARGS__))
#define FOR_EACH_APPLY4(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY3(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY2(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY1(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_APPLY0(fn, ...) fn(__VA_ARGS__)
#define FOR_EACH_FIRST(el, ...) el
#define FOR_EACH_TAIL(el, ...) __VA_OPT__(, __VA_ARGS__)
#define FOR_EACH_RESULT(fn, res, ...) res
// Right fold
#define FOR_EACH_R(fn, ...) __VA_OPT__(FOR_EACH_R_APPLY(FOR_EACH_L, fn, REVERSE(__VA_ARGS__)))
#define FOR_EACH_R_APPLY(fn, ...) fn(__VA_ARGS__)
// For testing
#define MY_FUNC(nested, var) (nested | var)
#define NEST_RECURSIVE(...) FOR_EACH_R(MY_FUNC, __VA_ARGS__)
NEST_RECURSIVE(A, B, C, D, E, F, G)