iOS：为什么我不能将 nil 设置为 NSDictionary 值？
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iOS: Why can't I set nil to NSDictionary value?
提问by applefreak
This might be very basic question but I was wondering why can't I assign nil as NSDictionary value? I have following statement many places in my code. If [q objectForKey:@"text"]is nil then App is crashing.
这可能是非常基本的问题，但我想知道为什么我不能将 nil 指定为 NSDictionary 值？我的代码中有很多地方都有以下语句。如果[q objectForKey:@"text"]为零，则应用程序崩溃。
NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:2];
[dict setObject:[q objectForKey:@"text"] forKey:@"text"];
I have to check everywhere for the nil before assigning it to dictionary. Is this the only correct way of doing? Am I missing something obvious?
在将其分配给字典之前，我必须到处检查 nil。这是唯一正确的做法吗？我错过了一些明显的东西吗？
if([q objectForKey:@"text"] != nil)
[dict setObject:[q objectForKey:@"text"] forKey:@"text"];
else
[dict setObject:@"" forKey:@"text"];
回答by David R?nnqvist
You canset a nil value using setValue:forKeybut it removes the key.
您可以使用设置一个 nil 值，setValue:forKey但它会删除键。
If you want to be able to set a key to nilyou could use setValue:forKey:which will remove the key if you set it to nil(quote from documentation below). Note the Valueinstead of Object.
如果您希望能够设置一个密钥，nil您可以使用setValue:forKey:它，如果您将其设置为nil（引用以下文档）将删除该密钥。注意Value而不是 Object。
setValue:forKey:Adds a given key-value pair to the dictionary.
Discussion...This method adds value and key to the dictionary using
setObject:forKey:, unless value isnilin which case the method instead attempts to remove key usingremoveObjectForKey:.
setValue:forKey:将给定的键值对添加到字典中。
讨论...此方法使用 将值和键添加到字典中
setObject:forKey:，除非值是nil这种情况，否则该方法会尝试使用 删除键removeObjectForKey:。
When you later try and get the object using objectForKey:for the key that you removed by setting it to nilyou will get nilback (quote from documentation below).
当您稍后尝试objectForKey:通过将其设置为nil您删除的键来获取对象时，您将nil返回（引用自下面的文档）。
Return value:The value associated with aKey, or nil if no value is associated with aKey.
返回值：与 aKey 关联的值，如果没有与 aKey 关联的值，则为 nil。
Note: The key will not actually be present in the dictionary so it won't be obtained using allKeys; or be enumerated over.
注意：该键实际上不会出现在字典中，因此不会使用allKeys;获取它。或一一列举。
回答by SamSol
You can set nil object in this way:
您可以通过这种方式设置 nil 对象：
NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
dictionary[@“key”] = nil;
Have you noticed it?
你注意到了吗？
NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
/* this statement is safe to execute */
dictionary[@“key”] = nil;
/* but this statement will crash application */
[dictionary setObject:nil forKey:@"key"];
回答by Avi Levin
When using this method:
使用此方法时：
func setObject(_ anObject: Any, forKey aKey: NSCopying)
Parameters (according to Apple doc's):
参数（根据 Apple 文档）：
anObject:
对象：
Raises an invalidArgumentException if anObject is nil. If you need to represent a nil value in the dictionary, use NSNull .
如果 anObject 为 nil，则引发 invalidArgumentException。如果需要在字典中表示一个 nil 值，请使用 NSNull 。
aKey
关键
Raises an invalidArgumentException if aKey is nil.
如果 aKey 为零，则引发 invalidArgumentException。
回答by Kunal Balani
My friend using nil as marker is a sign of bad programming . nil is reserved for some diffrent purpose .
我的朋友使用 nil 作为标记是编程不良的标志。nil 保留用于某些不同的目的。
if([q objectForKey:@"text"] != nil)
[dict setObject:[q objectForKey:@"text"] forKey:@"text"];
else
[dict removeObjectforKey:@"text"]; // this will do nothing if key does not exsist.
//by default for all the keys the value is nil and you are trying to override this behavior. going against the language rules will always get you in trouble .
//默认情况下，所有键的值都是 nil 并且您正在尝试覆盖此行为。违反语言规则总是会给你带来麻烦。
to check just use
检查只是使用
if([dict objectforKey:@"text"] !=nil){} // this will work becuase default value is nil
itself
