NSString 初始化
这里是 Objective-C 菜鸟。
为什么会这样：
NSString *myString = [NSString alloc];
[myString initWithFormat:@"%f", storedNumber];
导致以下异常 -length 仅为抽象类定义。定义 -[NSPlaceholderString length]!
当这工作正常时：
NSString *myString = [[NSString alloc] initWithFormat:@"%f",storedNumber];
我认为后者只是前者的缩写（但我显然错了，至少根据编译器来说）。
Objective-C noob here.
Why would this:
NSString *myString = [NSString alloc];
[myString initWithFormat:@"%f", storedNumber];
results in the following exception -length only defined for abstract class. Define -[NSPlaceholderString length]!
When this works just fine:
NSString *myString = [[NSString alloc] initWithFormat:@"%f", storedNumber];
I would think that the latter is merely a contraction of the former (but I'm obviously wrong, at least according to the compiler).
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因为
-initWithFormat:返回的对象与+alloc返回的对象不同，即与myString指向的对象不同>。这就是为什么您应该始终将+alloc与-init...结合起来的原因。这种情况在
NSString等类集群中很常见。+alloc返回一个通用字符串对象，然后-initWithFormat:决定NSString的具体子类，释放由创建的当前对象+alloc，从 NSString 的具体子类创建一个新对象，然后返回这个新对象。Because
-initWithFormat:is returning an object that’s different from the one returned by+alloc, i.e., an object that’s different from the one pointed bymyString. That’s the reason why you should always couple+allocwith-init….This situation is common in class clusters such as
NSString.+allocreturns a generic string object, then-initWithFormat:decides upon a concrete subclass ofNSString, deallocates the current object created by+alloc, creates a new object from a concrete subclass ofNSString, and then returns this new object.或者
or