Lifespan of NSString cStringUsingEncoding return value

问题

I have an NSTextField and get its contents like so

NSString *s = [textField stringValue]

Now I want to convert this NSString to a string that my platform-independent C code can handle. Thus I'm doing:

const char *cstr = [s cStringUsingEncoding:NSISOLatin1StringEncoding];

What I don't understand now is how long this "cstr" pointer stays valid. Apple docs for cStringUsingEncoding say:

The returned C string is guaranteed to be valid only until either the receiver is freed, or until the current memory is emptied, whichever occurs first. You should copy the C string or use getCString:maxLength:encoding: if it needs to store the C string beyond this time.

Two questions about this:

1. I suppose the aforementioned "receiver" is the NSString returned by the [textField stringValue]. Since I don't own this NSString how can I tell when this will be freed? Is it safe to assume that this NSString won't be freed before the NSTextField widget will be freed?

2. What does "until the current memory is emptied" mean precisely? I don't understand this at all.

Of course, I could just go ahead and make a copy but I'd like to understand how long the string pointer returned by cStringUsingEncoding is valid.

I know there are several similar questions here but none could really answer my question since in my case, the owner of the NSString is the NSTextField widget and I don't know when this widget will release the NSString or if it stays valid for the complete lifespan of the widget itself.

回答1:

I suppose the aforementioned "receiver" is the NSString returned by the [textField stringValue]

yes, in this case the receiver is s

Since I don't own this NSString how can I tell when this will be freed?

you don't. you should retain s by storing it in an instance variable for as long as you need it

Is it safe to assume that this NSString won't be freed before the NSTextField widget will be freed?

no, because you don't know what or how the text field returned s to you

What does "until the current memory is emptied" mean precisely? I don't understand this at all.

good question. also, hard to tell, because you don't own the string or know about its underlying implementation. say it was a mutable string that was mutated and had to reallocate memory...

you can be pretty sure of your safety if you copy s, store the copy in an instance variable and then use the copy to get the C string (or just copy the C string).

回答2:

Receiver for sure means the string s, and the danger to cstr is clear when s is freed. I think the phrase "or until current memory is emptied" is a documentation bug introduced by ARC. It can be read as "or until an ARC-implied release is executed".

See the doc quoted here in 2010 as evidence. I think the author, probably searching for 'autorelease pool' for places to update the docs, was grasping for a harmless, ARC-compatible synonym for "or until the current autorelease pool is emptied". I think it would have been better to just drop the disjunction.

Anyway, either take control of the NSString, or copy the cstring.

来源：https://stackoverflow.com/questions/29473872/lifespan-of-nsstring-cstringusingencoding-return-value