OBJECTIVE C NULL定義
在程式裡
給與變數
a=NULL 等於 a=nil 就是沒有占任何記憶體
而這時取aCustNo.length 會為0
如果是給ＮＵＬＬ物件
要打a=(id)[NSNull null]
但是如果如果取aCustNo.length 會產生錯誤
順便一提
textField 初始值不是(id)[NSNull null]
所以可以用aCustNo.length判斷
參考資料
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/NumbersandValues/Articles/Null.html
Using NSNull
The NSNull class defines a singleton object you use to represent null values in situations where nilis prohibited as a value (typically in a collection object such as an array or a dictionary).
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NSNull *nullValue = [NSNull null];
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NSArray *arrayWithNull = @[nullValue];
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NSLog(@"arrayWithNull: %@", arrayWithNull);
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// Output: "arrayWithNull: (<null>)"
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It is important to appreciate that the NSNull instance is semantically different from NO or false—these both represent a logical value; the NSNull instance represents the absence of a value. TheNSNull instance is semantically equivalent to nil, however it is also important to appreciate that it is not equal to nil. To test for a null object value, you must therefore make a direct object comparison.
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id aValue = [arrayWithNull objectAtIndex:0];
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if (aValue == nil) {
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NSLog(@"equals nil");
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}
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else if (aValue == [NSNull null]) {
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NSLog(@"equals NSNull instance");
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if ([aValue isEqual:nil]) {
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NSLog(@"isEqual:nil");
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}
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}
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// Output: "equals NSNull instance"
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© 2008 Apple Inc. All Rights Reserved. (Last updated: 2008-02-08)
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