EG:
File.h
UIKIT_EXTERN NSString *const SEARCH_KEY_INDEXPATH;
File.m
NSString * const SEARCH_KEY_INDEXPATH = @"SEARCH_KEY_INDEXPATH";
四種寫(xiě)法：
static const NSString *HSCoder = @"漢斯哈哈哈";
const NSString *HSCoder = @"漢斯哈哈哈";
NSString const *HSCoder = @"漢斯哈哈哈";
NSString * const HSCoder = @"漢斯哈哈哈";
全局常量：不管你定義在任何文件夾暂筝，外部都能訪問(wèn)
const NSString *HSCoder = @"漢斯哈哈哈";
局部常量：用static修飾后惋鸥，不能提供外界訪問(wèn)
static const NSString *HSCoder = @"漢斯哈哈哈";
