RAC中宏的使用-預編譯確定函數參數個數
對於RAC框架中的宏定義，預編譯期確認參數個數
/**
* Returns the number of arguments (up to twenty) provided to the macro. At
* least one argument must be provided.
*
* Inspired by P99: http://p99.gforge.inria.fr
*/
#define metamacro_argcount(...) \
metamacro_at(20, __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
metamacro_argcount是如何在預編譯期進行獲取參數個數的？
使用的宏有
-
##字符拼接A ## B，預編譯時就會處理爲AB -
__VA_ARGS__代表宏定義中的可變數量參數 ，即將左側的...處輸入的參數，對應到__VA_ARGS__位置
即在不確認參數的情況下，能夠將不確定的參數表示爲一個確定的__VA_ARGS__來呈現，支持C99規範的gcc編譯器纔有__VA_ARGS__這個宏。
正因爲這個宏，我們能夠在預編譯期來獲取參數的個數。下面看下是如何實現的。
/**
* Returns A and B concatenated after full macro expansion.
*/
#define metamacro_concat(A, B) \
metamacro_concat_(A, B)
/**
* Returns the Nth variadic argument (starting from zero). At least
* N + 1 variadic arguments must be given. N must be between zero and twenty,
* inclusive.
*/
#define metamacro_at(N, ...) \
metamacro_concat(metamacro_at, N)(__VA_ARGS__)
/**
* Returns the number of arguments (up to twenty) provided to the macro. At
* least one argument must be provided.
*
* Inspired by P99: http://p99.gforge.inria.fr
*/
#define metamacro_argcount(...) \
metamacro_at(20, __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
#define metamacro_stringify_(VALUE) # VALUE
#define metamacro_concat_(A, B) A ## B
#define metamacro_foreach_iter(INDEX, MACRO, ARG) MACRO(INDEX, ARG)
#define metamacro_head_(FIRST, ...) FIRST
#define metamacro_tail_(FIRST, ...) __VA_ARGS__
#define metamacro_consume_(...)
#define metamacro_expand_(...) __VA_ARGS__
// implemented from scratch so that metamacro_concat() doesn't end up nesting
#define metamacro_foreach_concat_iter(INDEX, BASE, ARG) metamacro_foreach_concat_iter_(BASE, ARG)
#define metamacro_foreach_concat_iter_(BASE, ARG) BASE ## ARG
// metamacro_at expansions
#define metamacro_at0(...) metamacro_head(__VA_ARGS__)
#define metamacro_at1(_0, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at2(_0, _1, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at3(_0, _1, _2, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at4(_0, _1, _2, _3, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at5(_0, _1, _2, _3, _4, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at6(_0, _1, _2, _3, _4, _5, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at7(_0, _1, _2, _3, _4, _5, _6, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at8(_0, _1, _2, _3, _4, _5, _6, _7, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at9(_0, _1, _2, _3, _4, _5, _6, _7, _8, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at10(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at11(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at12(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at13(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at14(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at15(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at16(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at17(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at18(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at19(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at20(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, ...) metamacro_head(__VA_ARGS__)
最終其實就是這兩個的對比：
參數中固定有20個值，當不確定的參數輸入時，假如__VA_ARGS__輸入了3個值，那麼總共有23個值
metamacro_at(20, __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
最終會轉換成：
metamacro_at20(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, ...) metamacro_head(__VA_ARGS__)
這就意味着前20個參數是不會通過...傳入__VA_ARGS__，那麼就會將後面3個參數傳入__VA_ARGS__，又因爲定義時以逆序20至1來排列，最終傳入的參數就是3.2.1，然後通過metamacro_head_(FIRST, ...) FIRST取第一個值，就得到了可變參數數量。
這個方法也就只適用於20個參數以內的情況，當然已經滿足的大部分的需求。
參考文章 https://blog.sunnyxx.com/2014/03/06/rac_1_macros/