iOS中一个可以获得参数个数的宏

Kael 发表于   |  

metamacro_argcount(…)

metamacro_argcount一个可以获得传入参数个数的Macro,

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metamacro_argcount(NSObject, version); 结果为2.
metamacro_argcount(NSObject, version, super); 结果为3
metamacro_argcount(NSObject, version, nil); 结果为3

这个宏在编译期(并非运行期)就获得参数个数,是不是很神奇?来看看里面有什么鬼

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//RACmetamacros.h中
#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)

需要展开下,以下就metamacro_argcount(NSObject, version)做展开

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metamacro_argcount(NSObject, version)
-->
metamacro_at(20, NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)

metamacro_at(N, ...)定义:

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#define metamacro_at(N, ...) \
        metamacro_concat(metamacro_at, N)(__VA_ARGS__)

继续展开

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metamacro_at(20, NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
-->
 metamacro_concat(metamacro_at,20)(NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);

metamacro_concat定义:

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#define metamacro_concat(A, B) \
        metamacro_concat_(A, B)
#define metamacro_concat_(A, B) A ## B

宏定义中A ## B,##对A,B分割,再将A,B代入拼接,
所以上文中`metamacro_concat(metamacro_at,20)就变成了metamacro_at20

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metamacro_concat(metamacro_at,20)(NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);
-->
metamacro_at20(NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);

metamacro_at20又是一个宏定义
metamacro_at##Nth定义:

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#define metamacro_at0...
...
#define metamacro_at19...
#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__)

继续转化 = =…

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NSObject对应_0
version对应_1
20对应_2
...
3对应_19

还剩(2,1)就是metamacro_head(__VA_ARGS__)的参数
所以

metamacro_at20(NSObject, version, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1);
-->
metamacro_at20(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, ...) metamacro_head(2,1)

metamacro_head定义:

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#define metamacro_head(...) \
        metamacro_head_(__VA_ARGS__, 0)
#define metamacro_head_(FIRST, ...) FIRST

再转化

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metamacro_head(2,1)
-->
metamacro_head_(2,1,0) 2
FIRST==2

经过各种展开

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metamacro_argcount(NSObject, version)
-->
metamacro_head(2,1)
-->
2

硬是获得了参数个数2……

大家都很熟悉数组,用数组来类比下metamacro_argcount的功能

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NSArray *array20=@[@20, @19, @18, @17, @16, @15, @14,@13, @12, @11, @10, @9, @8, @7, @6, @5, @4, @3, @2, @1,@0];
array20就取第20个
array20[20]=0默认没有参数

当我有参数的时候,我就插入到数组前面,取值就只取第20个元素
插入NSObjcet

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NSArray *array20=@[NSObjcet,@20, @19, @18, @17, @16, @15, @14,@13, @12, @11, @10, @9, @8, @7, @6, @5, @4, @3, @2, @1,@0];
array20[20]的值就变成了@1,就是1个参数

再插入version

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NSArray *array20=@[NSObjcet,version,@20, @19, @18, @17, @16, @15, @14,@13, @12, @11, @10, @9, @8, @7, @6, @5, @4, @3, @2, @1,@0];
array20[20]的值就变成了@2,就是2个参数