Whenever I need to create a new NSString variable I always alloc and init it. It seems that there are times when you don't want to do this. How do you know when to alloc and init an NSString and when not to?
Whenever I need to create a new NSString variable I always alloc and init it.
No, that doesn't make sense.
The variable exists from the moment the program encounters the point where you declare it:
NSString *myString;
This variable is not an NSString. It is storage for a pointer to an NSString. That's what the * indicates: That this variable holds a pointer.
The NSString object exists only from the moment you create one:
[[NSString alloc] init];
and the pointer to that object is only in the variable from the moment you assign it there:
myString = [[NSString alloc] init];
//Or, initializing the variable in its declaration:
NSString *myString = [[NSString alloc] init];
Thus, if you're going to get a string object from somewhere else (e.g., substringWithRange:), you can skip creating a new, empty one, because you're just going to replace the pointer to the empty string with the pointer to the other one.
Sometimes you do want to create an empty string; for example, if you're going to obtain a bunch of strings one at a time (e.g., from an NSScanner) and want to concatenate some or all of them into one big string, you can create an empty mutable string (using alloc and init) and send it appendString: messages to do the concatenations.
You also need to release any object you create by alloc. This is one of the rules in the Memory Management Programming Guide.
If you want to initialise it to a known value, there is little point in using alloc, you can just use a string literal:
NSString* myStr = #"Some value";
If you want to initialise it with a format or whatever, but don't need it to stick around beyond the current autorelease pool lifetime, it's a bit neater to use the class convenience methods:
NSString* myTempStr = [NSString stringWithFormat:#"%d", myIntVar];
If you need its lifetime to go beyond that, either alloc/init or else add a retain to the previous call. I tend to slightly prefer the latter, but the two are pretty much equivalent. Either way you will need a balancing release later.
Note that, since NSString is not mutable, this sort of thing is not only unnecessary but actively wrong:
// don't do this!
NSString* myStr = [[NSString alloc] initWithString:#""];
myStr = someOtherStr;
since it leaks the initial placeholder value.
It seems that there are times when you don't want to do this.
I can't think of any time when I would want to alloc/init a NSString. Since NSStringgs are immutable, you pretty much always create new strings by one of:
convenience class method e.g.
NSString* foo = [NSString stringWithFormat:...];
literal
NSString* foo = #"literal";
NSString instance method
NSString* foo = [bar uppercaseString];
copy from mutable string
NSString* foo = [mutableBar copy]; // foo needs to be released or autoreleased in this case
I'm guessing that you are referring to using StringWithString or similar instead of initWithString? StringWithString alloc and inits for you under the hood and then returns an autoreleased string.
If you don't need to do any string manipulation other than to have the string, you can use NSString *str = #"string";
In general with iOS, the tighter you manage your memory the better. This means that if you don't need to return a string from a method, you should alloc init and then release it.
If you need to return a string, of course you'll need to return an autoreleased string. I don't think its any more complicated than that.
Related
Actually I am working on a project with ARC enabled. I know using alloc and init is taking ownership of the object. I know, If I create a string like this
NSString *myString = [[NSString alloc]initWithFormat:#"Something"];
then I need to release the myString on myself. What If I am using ARC enabled? I cannot release on myself. So will it create a leak? Or should I don't create object like this?
I can create a string like below code too.
NSString *myString = [NSString stringWithFormat:#"Something"];
But which type I need to use exactly for ARC enabled project ? What will happen if I use first type?
If you use ARC, all the necessary release calls will be added for you when you compile. It will not leak.
The difference between
NSString *myString = [[NSString alloc]initWithFormat:#"Something"];
and
NSString *myString = [NSString stringWithFormat:#"Something"];
is that the first one will automatically released after the last reference to myString in that block, while the second one is an autoreleased instance that will only be released at the end of the run loop. It's not a big difference, but if you're using a lot of objects, try to avoid autoreleased ones to keep memory usage low.
ARC takes care of the memory management, so no you don't need to worry about calling release on your myString variable, ARC will do that for you. Also as a suggestion I would recommend using convenience methods to create your object such as
[NSString stringWithFormat:#"Something"];
It's enough to set the string pointer to nil to release it.
You can also do the same things that you would be able to do without ARC, but with the advantage that if you don't explicitly do anything, the ARC will manage (almost) everything for you.
So to release it you set it to nil, let's see what else you could do:
NSString* str= [[NSString alloc]initWithUTF8String: "Hello"];
// here the retain count of str is 1
__unsafe_unretained NSString* string= str;
// again 1 because string is __unsafe_unretained
void* data= (__bridge_retained void*) string;
// data retains the string, so the retain count is to 2
// This is useful in the case that you have to pass an objective-c object
// through a void pointer.You could also say NSString* data= string;
str=nil;
// Here the retain count of str is 1
NSLog(#"%#",(__bridge NSString*)data);
UPDATE
Here's why sometimes you don't notice that an object is released:
NSString* str= [[NSString alloc]initWithString: #"hey"];
__unsafe_unretained NSString* str2=str;
str=nil;
NSLog(#"%#",str2);
In this case str=[[NSString alloc]initWithString: #"hey"] is equal to str=#"hey", with the difference that str is autoreleased and not released.But the compiler optimizes the code in str=#"hello", so if you are inside an autorelease block you won't have any problem, str2 will be printed correctly.
That's why I used initWithUTF8String, to avoid that compiler optimization.
When I alloc and init two NSString variables and compare their pointers, they are the same. Here's a snippet that shows this:
NSString *s1 = [[NSString alloc] initWithString:#"hello world"];
NSString *s2 = [[NSString alloc] initWithString:#"hello world"];
if (s1 == s2) {
NSLog(#"==");
}else {
NSLog(#"!=");
}
Why are s1 and s2 the same?
There are three things going on here:
Firstly, the two identical string literals you're passing in to initWithString: will have the same address to start. This is an obvious optimization for constant data.
Secondly, when you nest alloc and init with strings, the runtime performs an optimization, the alloc call essentially becomes a no-op. This is done using the NSPlaceholderString class. This means the pointer you get back here will be coming from initWithString:, not from alloc.
Thirdly, under the hood, initWithString: is calling CFStringCreateCopy, which as you may find, has the following behavior: Since this routine is for creating immutable strings, it has an optimization. It simply calls CFRetain() and returns the same object that was passed in.
Thanks for the very interesting question. I had fun figuring it out.
#"hello world" strings are of class NSConstantString.if you use #"hello world" in two places, they will be referencing the very same object.
From documentation.
The simplest way to create a string object in source code is to use
the Objective-C #"..." construct:
NSString *temp = #"/tmp/scratch"; Note that, when creating a string
constant in this fashion, you should use UTF-8 characters. Such an
object is created at compile time and exists throughout your program’s
execution. The compiler makes such object constants unique on a
per-module basis, and they’re never deallocated, though you can retain
and release them as you do any other object. You can also send
messages directly to a string constant as you do any other string:
BOOL same = [#"comparison" isEqualToString:myString];
If I set a NSString as property
#property (copy) NSString* name;
I always want to use (copy), so that if its value change, all object with such string still have the old value (as specifiedhere).
However, what happens if my NSString is not a class property, but it is just declared in the code ? Is in that case retained or copied everytime I assign a new value ?
thanks
It depends on how you declare it. You should read the documentation on memory management. Basically the rules are:
NSString *aString = [NSString stringWithString:#"Hello"];
NSString *bString = [NSString stringWithFormat:#"%#", #"Hello"];
In these cases, the string is not copied or retained. It is autoreleased, which means it will be automatically deallocated the next time the autorelease pool drains. You do not have to call the release method on them. (So assigning a new value will not cause it to leak.)
NSString *cString = [[NSString alloc] initWithFormat:#"%#", #"Hello"];
[cString release];
According to Objective C convention, methods that use alloc and have a retain count of one are not autoreleased, so you need to release them explicitly. Assigning a new value without releasing the old one will cause a leak.
You can also explicitly call a "copy" method or a "retain" method on a string. In either case, the new string will have a retain count of 1 and will not be autoreleased, so you will need to call the release method on it before you assign a new value.
NSString *dString = [cString retain];
NSString *eString = [cString copy];
...
[dString release];
[eString release];
If it is a property, and you use self.variableName, this will be taken care of for you (through the getters and setters which are generated with #synthesize). If you do it explicitly, you must make sure to call release on variables that you have called retain or copy on.
Edit: As some commentators below have noted, thinking about management in terms of "ownership" is usually the preferred of describing these ideas, rather than retain count.
If it's not a property and just declared in code, you need to explicitly retain or copy it, ie
NSString myString = [otherString copy];
or
NSString myString = [otherString retain];
Either way you also need to ensure it's released at somepoint.
If you're not using the property's setter, like self.name = #"foo" or [self setName:#"foo"], but rather assign the variable directly, like name = #"foo", it doesn't matter at all how the property is declared.
You have to understand that the property syntax is just a shortcut for writing accessor methods (-name and -setName: in this case). If you're not calling these methods (implicitly by setting the property), it doesn't matter how they work internally (which is what you specify by retain or copy).
I am kind of confused by the behavior of NSString *str..
I assigned it in several ways, sometimes it works, and sometimes it becomes null.
NSString *str = #"/hi/hello"; // this one always works
// this sometimes becomes null after the function ends
NSString *str2 = [str lastPathComponent];
// as above
NSString *str3 = [NSString stringWithString:str2];
NSString *str4 = [NSString initWithString:str3];
I am not quite familiar with the object behavior of Obj-C, is it just like C++?
If so, how can I do assignment safely like
string str = "hi";
string str2 = str;
behaves in C++?
ex: I declare a string in my .h file,
how to assign it safely that it wouldn't become NULL after a function ends?
I know it's a very basic question, but I can't find the answer in NSString reference page.
Really thanks for any help!
The behaviour is not just like C++. Objects are reference-counted. If you want to keep one around, you must place a claim on it.
If you create the object yourself with a method whose name includes the word alloc, new or copy then you have ownership already. This is like a C++ new. (When you have created an object with alloc, you need also to initialise it with some method whose name begins init. But you have to create it first. In C++ both things would be considered parts of the single act of construction.)
Objects you receive from other methods (such as two of the three NSString methods you mention) are only transiently available unless you explicitly claim ownership by calling [object retain]. You only need to do this if you want to keep them around beyond the immediate context. (There isn't really an equivalent to this in C++.)
However you gain ownership, you must relinquish it when you are finished by calling [object release]. This sort of like a C++ delete, except that the object isn't actually destroyed until all ownership claims are released.
Getting to grips with this is really really really important, perhaps the only important thing you need to know to use Objective-C. Read the object ownership documentation carefully and you'll be sorted.
I assume you're not using garbage collection? If this is the case then you need to retain the string.
NSString* str2 = [[str lastPathComponent] retain];
I suggest you do some reading on objective-c memory management.
NSString *str = #"/hi/hello";
This works because it creates a string literal. Answers to this question are worth a read to understand these in Objective-C
What's the difference between a string constant and a string literal?
In all these cases you are creating autoreleased strings. These will be deallocated when you return to the application's runloop.
NSString *str2 = [str lastPathComponent];
NSString *str3 = [NSString stringWithString:str2];
In this last one I assume you meant [[NSString alloc] initWithString:str3]
This creates a string that is retained. But this isn't a good way to create static strings.
You should create static strings in your implementation file like this
static NSString *myConstant = #"constantString"
As NSString strings are immutable, what is the value of the stringWithString: class method?
I get the utility when used with NSMutableString, I just didn't see the utility with the NSString class.
You might have a NSMutableString (or some home-grown NSString subclass) that you want to duplicate.
NSMutableString *buffer = [NSMutableString string];
// do something with buffer
NSString *immutableStringToKeepAround = [NSString stringWithString:buffer];
Of course, you can also just make a copy:
NSMutableString *buffer = [NSMutableString string];
// do something with buffer
NSString *immutableStringToKeepAround = [[buffer copy] autorelease];
but you own the copy and must release or autorelease it.
As "Andy" points out in #318666, it's related to memory management, quoting:
The difference between initWithString and stringWithString is that stringWithString returns an auto-released pointer. This means that you don't need to release it specifically, since that will be taken care of next time that the auto-release pool cleans up any auto-released pointers.
initWithString, on the other hand, returns a pointer with a retain count of 1 - you do need to call release on that pointer, or else it would result in a memory leak.
(Taken from here)
Returns a string created by copying the characters from another given string
[NSString stringWithString:#"some string"]
It is equivalent to
[[[NSString alloc] initWithString:#"some string"] autorelease]
Also, if you have a pointer to an NSString, it may actually be a subclass of NSString like NSMutableString. So, if you want to store the string and be guaranteed that it doesn't change, you should make a copy of it, hence stringWithString exists.
As another use case, if (for whatever reason) you create your own subclass of NSString or NSMutableString, stringWithString: provides a handy way to instantiate it with an instance of either NSString, NSMutableString, or MyCustomString.
I often use +stringWithString: when I need to create an NSMutableString but start it with an initial value. For example:
NSMutableString * output = [NSMutableString stringWithString:#"<ul>"];
for (NSString * item in anArray) {
[output appendFormat:#"<li>%#</li>", item];
}
[output appendString:#"</ul>"];
FYI, now that we are compiling with ARC enabled, you don't have to manually release at all, ARC will insert the release calls during compile time. So how is it still different? stringWithString is still added to the autorelease pool which gets drained sometime in the future (unless you created your own autorelease pool). initWithString will have a release call right before the function ends, so if you didn't retain it somewhere in the method, you can be sure that the string is destroyed by the end of the function call. This gives you a tighter control on the memory management as opposed to using autoreleased objects.