EG:

File.h

UIKIT_EXTERN NSString *const SEARCH_KEY_INDEXPATH;

File.m

NSString * const SEARCH_KEY_INDEXPATH = @"SEARCH_KEY_INDEXPATH";

四种写法：

static const NSString *HSCoder = @"汉斯哈哈哈";
const NSString *HSCoder = @"汉斯哈哈哈";
NSString const *HSCoder = @"汉斯哈哈哈";
NSString * const HSCoder = @"汉斯哈哈哈";

全局常量：不管你定义在任何文件夹，外部都能访问

const NSString *HSCoder = @"汉斯哈哈哈";

局部常量：用static修饰后，不能提供外界访问

static const NSString *HSCoder = @"汉斯哈哈哈";