I understand that the v03 example creates an object that I own and must ultimately release. What I would like to know is are there any differences between the first two(v_01 & v02), or are they essentially the same?
// Version_01
userName = @"Teddy";
// Version_02
userName = [NSString stringWithString:@"Gary"];
// Version_03
userName = [[NSString alloc] initWithString:@"Caroline"];
... some code
[userName release];
gary
The first two or essentially the same
The latter one is not however as userName would be an invalid string object at that point.
However changing it to
userName = [[[NSString alloc] initWithString:@"Caroline"] autorelease];
would render it effectively the same as the previous two.
version 01: is a string constant.... version 02: is an auto-released NSString created by copying a constant.
They are all similar, but there are some slight differences between the three.
The first one is a pointer to a string constant. The string Teddy is stored in read-only memory, and userName is a pointer to this string constant. You need not (and cannot) retain or release this object, since it exists "permanently" (that is, for the duration of the program).
The second one is an autoreleased string object with the contents Gary. When returned to you, it has a release count of 0. It may be retained and released as needed.
The third one is similar to the second one, but it is not autoreleased, so it has a retain count of 1 when it is initially returned to you. Like the second one, it may be retained and released as needed.
Most of the differences in these instances is how the memory is managed. If you want a clearer view of what's happening in the background, you might want to peruse the Objective-C Memory Management Guide.
// Version_01
userName = @"Teddy";
This is a String constant that does not have any memory management associated with it. The memory used to hold the value is part of the memory in which the code resides in (essentially). retain and release calls on the variable will be ignored.
// Version_02
userName = [NSString stringWithString:@"Gary"];
This is an autoreleased instance of an NSString object. Its retain count is currently one and will be released by the autorelease pool soon unless it is retained.
// Version_03
userName = [[NSString alloc] initWithString:@"Caroline"];
[userName release];
This is a managed instance of an NSString. When it is first initialized, its retain count is one. After releasing it, the retain count went down to zero, therefore, its memory will be deallocated. Referring to the variable userName after releasing it will cause an EXE_BAD_ACCESS error.
Practically, there are no differences, as long as you don't attempt to use the Version_03 string after you release it. If you want to do that, you'll need to autorelease it instead.