RAC宏分析5：metamacro_concat

metamacro_concat
这个宏也很简单，连接而已：

/**
 * Returns A and B concatenated after full macro expansion.
 */
#define metamacro_concat(A, B) \
        metamacro_concat_(A, B)

#define metamacro_concat_(A, B) A ## B
释义：拼接A和B，即AB
示例：
NSString *str = @"Hello china!";
NSLog(@"%@", metamacro_concat_(st, r));  // Hello china!