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如何NSString在Objective C中测试a是否为空？

我想知道的区别nil,NIL和null.我用Google搜索了一下,发现了这个:

nil - >指向Objective-C对象的null指针

NIL - >指向Objective-C类的null指针

null - > null指向原始类型或缺少数据的指针

但我无法清楚地理解术语"Objective-C对象"和"类".

请向我解释一下.此外,有没有像任何文字NSNull或NSNil在Objective-C？如果是这样,那么请解释它的用途.

我有一段代码检测如果NSString是NULL,nil等等.然而,它崩溃.这是我的代码:

NSArray *resultstwo = [database executeQuery:@"SELECT * FROM processes WHERE ready='yes' LIMIT 0,1"];
for (NSDictionary *rowtwo in resultstwo) {

NSString *getCaption = [rowtwo valueForKey:@"caption"];

if (getCaption == NULL) {
theCaption = @"Photo uploaded...";
} else if (getCaption == nil) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@""]) {
theCaption = @"Photo uploaded...";
} else if ([getCaption isEqualToString:@" "]) {
theCaption = @"Photo uploaded...";
}

}

这是错误:

因未捕获的异常而终止应用' NSInvalidArgumentException',原因:' -[NSNull isEqualToString:]:无法识别的选择器发送到实例 …