在这种情况下，NSMutableString比NSString更有效吗？ [英] Is an NSMutableString in this case more efficient than NSString?

问题描述

如果我以这种方式声明一个指针变量：

If I declare a pointer variable in this fashion:

NSString *foo;

然后在我的代码中的某处做一些事情，例如：

And then somewhere later in my code do something such as:

foo = @"bar";

这是最终采用我的NSString，在内存中创建一个新的副本与一个额外的字符串，然后删除我的初始字符串？

Is that ultimately taking my NSString, creating a new copy in memory with an additional string, then deleting my initial string? Would foo be better off as an NSMutableString?

推荐答案

不， foo 是一个指向 NSString 的指针的变量。分配 foo = @bar将指针 foo 存储的值设置为NSString的地址 @bar。没有拷贝。如果 foo 已经指向另一个 NSString 实例不是一个字符串constsant像 @bar），没有其他引用，那么你有一个内存泄漏。

No, foo is variable holding a pointer to an NSString. The assignment foo = @"bar" sets the value stored by the pointer foo to the address of the NSString @"bar". There is no copy made. If foo already pointed to an other NSString instance that was not a string constsant (i.e. like @"bar") and there are no other references to that instance, then you have a memory leak. You would

[foo release];
foo = @"bar";

您不需要保留或释放字符串常量，例如 @bar。

in that case. You do not need to retain or release string constants like @"bar".

字符串常量不能改变，所以如果你尝试修改常量字符串的值，你会得到一个运行时错误。将 @bar分配给 NSString * 与之间没有区别NSMutableString * 。当然，你不能使用 NSMutableString 的变异方法而没有运行时错误，只是因为你分配了@bar的地址（一个NSString实例）到 NSMutableString * 类型的变量。如果你想改变字符串，你会做

String constants cannot be mutated, so you will get a runtime error if you try to modify the value of a constant string. There's no difference between assigning @"bar" to an NSString* vs an NSMutableString*. Of course, you won't be able to use the mutating methods of the NSMutableString without a runtime error just because you assign the address of @"bar" (an NSString instance) to a variable of type NSMutableString*. If you want to mutate the string, you would do

NSMutableString *mutableFoo = [@"bar" mutableCopy];

在这种情况下，显然会有一个副本，并且您负责发布 mutableFoo 当你完成它。

In this case, a copy is obviously made and you are responsible for releasing mutableFoo when you're done with it.

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