附加到 NSString 时正确的内存管理

Question

我已经分配了一个 NSString 对象。我想在应用程序的整个生命周期中偶尔附加到该字符串 我对如何正确处理下面示例中的内存感到困惑。谢谢。

1. 我是否需要保留每次附加的字符串，或者是否可以附加属于自动释放池的字符串，如 stringByAppendingString: 返回？
2. 每次执行追加操作时，我是否都会泄漏之前分配给 my_string 的内存？

代码：

NSString* my_string = [[NSString alloc] initWithString:@"Initial string"];
NSString* something = [NSString stringWithFormat:@"%@", @" with something"];
my_string = [[my_string stringByAppendingString:something] retain];
NSString* something_else = [NSString stringWithFormat:@"%@", @" and something_else"];
my_string = [[my_string stringByAppendingString:something_else] retain];
[my_string release];

Answer 1

NSString* my_string = [[NSString alloc] initWithString:@"Initial string"];

不是自动释放的（因为它是使用 init... 方法创建的，这是约定），它的保留计数为 1。

NSString* something = [NSString stringWithFormat:@"%@", @" with something"];

是自动释放的。

是否可以附加属于自动释放池的字符串，如
stringByAppendingString：返回

只要您期望它存在就可以。但是在您的代码中：

NSString* my_string = [[NSString alloc] initWithString:@"Initial string"];
NSString* something = [NSString stringWithFormat:@"%@", @" with something"];
my_string = [[my_string stringByAppendingString:something] retain];

您有泄漏。您应该在重新分配 my_string 之前自动释放它。所以最后一行应该是：

my_string = [[[my_string autorelease] stringByAppendingString:something] retain];

否则你将永远失去指向你应该释放的对象的指针。当你保留结果时，你有责任释放它，所以你在最后一行之前也有泄漏，这应该是

my_string = [[[my_string autorelease] stringByAppendingString:something_else] retain];

Answer 2

首先要记住的是 NSString 是不可变的。无论您是分配、使用格式创建还是追加，您总是会返回一个新字符串对象。那么让我们来分解一下：

NSString* my_string = [[NSString alloc] initWithString:@"Initial string"];
//String object created with "Initial string" content. Retain +1

NSString* something = [NSString stringWithFormat:@"%@", @" with something"];
//String object created with content " with something" and autoreleased.

my_string = [[my_string stringByAppendingString:something] retain];
//String object created with content "Initial string with something", autorelesed, and retained. Effective retain +1
//Assignment replaces old "Initial string" reference. "Initial string" object still has retain of +1, so it doesn't get deallocated (it leaks).

NSString* something_else = [NSString stringWithFormat:@"%@", @" and something_else"];
//String object created with content " and something_else" and autoreleased.

my_string = [[my_string stringByAppendingString:something_else] retain];
//String object created with contents "Initial string with something and something_else", autoreleaased, and retained. Effective retain +1
//Assignment replaces old "Initial string with something" reference. "initial string with something" object still had a retain of +1, so the object leaks.

[my_string release];
//String object "Initial string with something and something_else" released. Retain -1. Object is deallocated and doesn't leak.

//...Sometime later at the end of the runloop, the autorelease pool is drained and " with something" and " with something_else" are both deallocated.

所以：

1. 是的。附加一个自动释放的字符串就可以了。在运行循环结束之前（至少是在该方法退出之后），不会对自动释放的对象调用 release。

2. 是的，每次执行追加时，您都会泄漏之前分配给 my_string 的对象...但导致泄漏的不是追加，而是分配。您可以将 nil 分配给 my_string，并且如果您不release，用于指向的保留对象 my_string 仍然会泄漏code> 或 autorelease 首先。

Answer 3

在第二个作业中，您将丢失第一个 stringByAppendingString 调用的保留引用，因此：

1. 可以在不调用保留的情况下附加字符串，除非您稍后在代码中需要该值。
2. 是的，每次调用 stringByAppendingString 都会返回一个新的自动释放字符串。如果您暂时使用该值，请勿保留它。