objective-c 使用常量 NSString 作为 NSUserDefaults 的键
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Using a constant NSString as the key for NSUserDefaults
提问by Olly
I'm using NSUSerDefaults to store user preferences. I remember reading somewhere that setting the keys as constants is a good idea - and I agree. The following code is what I currently have:
我使用 NSUSerDefaults 来存储用户首选项。我记得在某处读到将键设置为常量是个好主意 - 我同意。以下代码是我目前拥有的：
[[NSUserDefaults standardUserDefaults]
setObject:[NSNumber numberWithInt:polygon.numberOfSides]
forKey:@"polygonNumberOfSides"];
I tried changing this to:
我尝试将其更改为：
@implementation Controller
NSString const *kPolygonNumberOfSides = @"polygonNumberOfSides";
-(void)savePolygonInfo {
[[NSUserDefaults standardUserDefaults]
setObject:[NSNumber numberWithInt:polygon.numberOfSides]
forKey:kPolygonNumberOfSides];
}
While this does work, it produces "warning: passing argument 1 of 'objectForKey:' discards qualifiers from pointer target type". I'm keen to keep my code free from compiler warnings. How can I fix this warning?
虽然这确实有效，但它会产生“ warning: passing argument 1 of 'objectForKey:' discards qualifiers from pointer target type”。我很想让我的代码不受编译器警告的影响。我该如何解决这个警告？
回答by e.James
You should use:
你应该使用：
NSString * const kPolygonNumberOfSides = @"..."; // const pointer
instead of:
代替：
NSString const * kPolygonNumberOfSides = @"..."; // pointer to const
The first is a constant pointer to an NSString object, while the second is a pointer to a constant NSString object.
第一个是指向 NSString 对象的常量指针，而第二个是指向常量 NSString 对象的指针。
It is a subtle difference. The compiler warning happens because setObject:forKey:is declared as follows:
这是一个微妙的区别。发生编译器警告setObject:forKey:是因为声明如下：
- (void)setObject:(id)value forKey:(NSString *)defaultName;
It is expecting the defaultNameargument to be of type NSString *. When you instead pass in a pointer to a constant, you've given it something different.
它期望defaultName参数的类型为NSString *。当你传入一个指向常量的指针时，你给了它一些不同的东西。
Update:I want to point out that these constants should be defined as staticif they are only going to be used from within a single file. I say this because I have run across this problem myself: if you do not declare them as static, then they will exist in the global namespace, and you will not be able to use a variable with the same the same name in another file. see Constants in Objective-Cfor more information. To explain by example, this is what I currently use for keys that I only need to use in one .mfile:
更新：我想指出这些常量应该被定义为static它们只在单个文件中使用。我这样说是因为我自己也遇到过这个问题：如果你不将它们声明为静态，那么它们将存在于全局命名空间中，并且你将无法在另一个文件中使用同名的变量。有关更多信息，请参阅Objective-C 中的常量。举例说明，这就是我目前用于只需要在一个.m文件中使用的密钥的方法：
static NSString * const kSomeLabel = @"...";
回答by Adam Rosenfield
Don't use constwith Objective-C objects, they weren't really designed to use it. NSStringobjects (among many others) are already immutable by default by virtue of their design, so making them constis useless.
不要const与 Objective-C 对象一起使用，它们并不是真正设计来使用它的。 NSString对象（以及许多其他对象）由于其设计而在默认情况下已经是不可变的，因此创建它们const是无用的。
As e.James suggested, you can use an NSString * const, which is a constant pointer to an NSString. This is subtly different from a const NSString *(equivalent to NSString const *), which is a pointer to a constant NSString. Using a NSString * constprevents you from reassigning kPolyto point to a new NSStringobject.
正如e.James 所建议的，您可以使用 an NSString * const，它是一个指向 an 的常量指针NSString。这与 a const NSString *（等价于NSString const *）略有不同，后者是指向常量 的指针NSString。使用 aNSString * const可以防止您重新分配kPoly指向新NSString对象。
回答by malhal
For access from other classes:
从其他类访问：
.h
。H
extern NSString * const PolygonNumberOfSidesPrefsKey;
.m
.m
NSString * const PolygonNumberOfSidesPrefsKey = @"PolygonNumberOfSides"
For access only inside current class:
仅在当前类中访问：
.m
.m
static NSString * const kPolygonNumberOfSidesPrefsKey = @"PolygonNumberOfSides"
回答by Abizern
I would suggest even making the constant more descriptive. A constant for the number of sides of a polygon could come from anywhere. As a suggestion, how about:
我建议甚至让常量更具描述性。多边形边数的常数可以来自任何地方。作为建议，如何：
kDefaultsPolygonNumberOfSides;
instead.
反而。
回答by Sean Patrick Murphy
For additional background about this issue, there is an excellent article on Wikipedia explaining the constant syntax with pointers: http://en.wikipedia.org/wiki/Const_correctness#Pointers_and_references
有关此问题的其他背景信息，维基百科上有一篇出色的文章用指针解释了常量语法：http: //en.wikipedia.org/wiki/Const_correctness#Pointers_and_references
