Mobile Application Programming. Memory Management

Transcription

1 Mobile Application Programming Memory Management

2 Memory Management

3 Ownership Model Memory Management in Objective-C is based on an ownership model. Objects may have many owners. Actions that result in the caller owning the object: Creating an object with alloc Retaining an object with retain Copy an object using copy Owners of objects must call release to relinquish ownership

4 Collections Collections such as NSMutableArray take ownership of objects when they are added to them using retain. After adding an object to a collection, you may release the object immediately. When the collection is released, it releases all of the objects it contains. NSString* stringnum1 = [[NSString alloc] initwithformat:@ %i, 25]; NSString* stringnum2 = [[NSString alloc] initwithformat:@ %i, 42]; NSMutableArray* array = [[NSMutableArray alloc] initwithcapacity:16]; [array addobject:stringnum1]; [array addobject:stringnum2]; [stringnum1 release]; [stringnum2 release]; [array release];

5 Autorelease There is a method like release called autorelease that calls release on an object at some point in the future The object will not be deallocated during the current function, but is not guaranteed to exist after the function finishes and control is returned to the system Usually autoreleased objects are released the next time program control returns to the operating system Think of autoreleased objects as local variables

6 Accessors It is common to receive an object from an accessor method such as: NSString* name = [person name]; This string object is returned to you as a shared object or after having been autoreleased. You do not own it and do not need to call release on it.

7 Convenience Constructors Constructors that do not use alloc return an object that has been autoreleased as well. These are called convenience constructors. Common examples: NSString* agecaption = [NSString stringwithformat:@ Age: %i, 25]; NSData* data = [NSData datawithcontentsoffile:@ SomeFile.dat ]; UIImage* image = [UIImage imagenamed:@ SomeImage.jpg ];

8 Ownership Model Own Object alloc copy retain Don t Own Object accessors convenience constructors released objects autoreleased objects

9 Pointers and Memory Pitfalls What is a pointer? A pointer is an integer variable whose value is an address in memory. Thats it! Using a pointer, another chunk of memory can be read or written to You ve been using them already! All Objective-C objects are accessed via a pointer. (eg. NSString*)

10 Pointers and Memory Pitfalls The reason why pointers are dangerous is because they can point to any memory location. Even locations that are not allocated to your program. Accessing (reading or writing) to memory pointed to by an invalid pointer can: Access memory that isn t yours, possibly causing a page fault Overwrite and corrupt other data in your program. This is a very nasty bug!

11 Pointers and Memory Pitfalls Why use pointers then? They can create very fast code Some problems are easier to think about in terms of changing pointer values Many APIs and libraries use them Because you have to in Objective-C to interact with your objects

12 Pointers and Memory Pitfalls Most Common Pointer Errors Uninitialized pointer NULL or nil pointer Pointer to previously freed memory (this one is potentially the most nasty!)

13 Avoiding Memory Errors Local Pointers to Classes: autoreleased at assignment PTPoint* p = [[[PTPoint alloc] initwithx:0.4 y:-0.7] autorelease]; NSString* s = [NSString stringwithformat:@ Some string %i, 5]; Class Pointers to Classes: retained at assignment _name = [[NSString stringwithformat:@ Some string %i, 5] retain]; _position = [[PTPoint alloc] initwithx:0.4 y:-0.7];

14 Summary Follow the ownership model Make objects last as long as the pointers that point to them. No longer, no shorter.