Objective-C Memory Management

Objective-C Memory Management

Using Reference Counting

每一个从NSObject派生的对象都继承了对应的内存管理的行为。这些类的内部存在一个称为retain count的计数器。使用对应的调用，这个计数器会做自增或者自减。Objective-C runtime知道这个计数器何时变为0，此时会做deallocated。当对应deallocated时，它所占用的所有memory都会被释放。
有三种方法可以让retain count增加：

当你使用含有"alloc","create"的api来创建对象时，这个对象的retain count为1；

另外，当你通过一个包含"copy"的方法来得到一个对象时，这个对象的retain count为1；

你可以通过手动调用"retain"方法来增加retain count；
当你要减小retain count时，你需要调用

release

。
A standard allocation of an object

Bar foo = [ [ Bar alloc ] init ];
Retaining an object*

Bar* foo = [ [ Bar alloc ] init ];
[ foo retain ];

Using autorelease

Returning an autoreleased object

-( Foo* )getFoo {
Foo* foo = [ [ Foo alloc ] init ];
//do something with foo here...
return [ [ foo autorelease ] ];
}

The alloc/autorelease pattern

-( void )someMethod {
Foo* foo = [ [ Foo alloc ] init ] autorelease ];

//foo is still vaild here,
//it won't be released until the method exists
[ foo doSomething ];
}

Using factory methods versus the traditional allocation pattern

-( void )usingFactories {
NSMutableArray* array = [ NSMutableArray array ];    //nice, simple, autoreleased.
NsMutableArray* array2 = [ [ NSMutableArray alloc ] init ];
// do stuff with array and array2...

// need to release this one.
[ array2 release ];

// [ array release ]; no need to release this,
// it's already autoreleased
// if you release it here, it will cause a crash
}

Creating your own autorelease pool

-( void )inflateMemoryUsage {
for( NSUInteger n = 0; n < 100000; ++n ) {
NSAutoreleasePool *pool = [ [ NSAutoreleasePool alloc ] init ];
//this object is autoreleased
NSdata* data = [ self getBigBlobofData ];
// do something with data...
[ self doStuff:data ];
[ pool release ];    // the autoreleased objects are deallocated here.
}
//nothing left over
}