类中的extern NSString * const. [英] extern NSString *const in a class.

问题描述

我有这个头文件:

#import <Foundation/Foundation.h>

@interface PCConstants : NSObject
extern NSString *const kPCUserProfileKey;
extern NSString *const kPCUserProfileNameKey;
extern NSString *const kPCUserProfileFirstNameKey;
extern NSString *const kPCUserProfileLocationKey;
extern NSString *const kPCUserProfileGenderKey;
extern NSString *const kPCUserProfileBirthDayKey;
extern NSString *const kPCUserProfileInterestedInKey;
@end

实现:

#import "PCConstants.h"

@implementation PCConstants

NSString *const kPCUserProfileKey                  = @"profile";
NSString *const kPCUserProfileNameKey              = @"name";
NSString *const kPCUserProfileFirstNameKey         = @"firstname";
NSString *const kPCUserProfileLocationKey          = @"location";
NSString *const kPCUserProfileGenderKey            = @"gender";
NSString *const kPCUserProfileBirthDayKey          = @"birthday";
NSString *const kPCUserProfileInterestedInKey      = @"interestedIn";

@end

当我在.pch文件中导入头文件时，我可以在任何地方访问常量.但是我试图了解发生了什么.

When I import the header file in my .pch file, I can access the constants everywhere. But I try to understand what's going on.

我从不分配初始化对象，所以它们不能是实例常量.因此，这些常量必须是类对象常量对吗?但是我认为类对象不能包含数据.

I never alloc init this object, so they can't be instance constants. So the constants must be "class object constants right? But I thought class objects could not contain data.

有人可以解释吗?

推荐答案

这些extern变量是应用程序级的全局变量.它们没有作用于该类，也没有作用于该类的实例.

Those extern variables are app-level globals. They are not scoped to the class and they are not scoped to an instance of the class.

Objective-C不支持实例或类级别的全局变量.

Objective-C does not support instance or class level globals.

如果要使用类级别的常量，则需要定义类方法来访问它们.如果需要实例级常量，则需要定义实例方法或只读属性以访问它们.

If you want class level constants, you need to define class methods to access them. If you want instance level constants, you need to define instance methods or read-only properties to access them.

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