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iOS Memory Management and Multithreading

Added: (Tue Nov 14 2017)

Pressbox (Press Release) - This post contains my perusing notes of the book , from the components of ARC to the use of Blocks and Grand Central Dispatch.

Memory Management Actions in Objective-C - Reference Counting

Activity for Objective-C Object Objective-C Method

Make and have responsibility for/new/duplicate/mutableCopy gathering

Take responsibility for

Surrender it release

Discard it dealloc



+ (id) alloc {

return [self allocWithZone: NSDefaultMallocZone()];


+ (id) allocWithZone: (NSZone*)z {

return NSAllocateObject (self, 0, z);


struct obj_layout {

NSUInteger held;


inline id

NSAllocateObject (Class aClass, NSUInteger extraBytes, NSZone *zone) {

int estimate =/* required size to store the question */

id new = NSZoneMalloc(zone, measure);

memset(new, 0, measure);

new = (id)&((struct obj_layout *)new)[1];


The NSAllocateObject work calls NSZoneMalloc to dispense a memory territory. From that point forward, the region is loaded with zero and the range pointer is returned.

Objective-C now overlooks zones, which was utilized to avoid memory discontinuity. The alloc strategy would now be able to be revised as:


struct obj_layout {

NSUInteger held;


+ (id) alloc {

int measure = sizeof(struct obj_layout) + size_of_the_object;

struct obj_layout *p = (struct obj_layout *)calloc(1, measure);

return (id)(p + 1);


The alloc strategy restores a memory square loaded with zero containing a struct obj_layout header, which has a variable "held" to store the quantity of references(reference check).


get reference tally an incentive by calling retainCount:


id obj = [[NSObject alloc] init];

NSLog(@"retainCount=%d", [obj retainCount]);


* retainCount=1 is shown. */

Execution of hold:


- (id) retain{


return self;


inline void NSIncrementExtraRefCount(id anObject) {

in the event that (((struct obj_layout *)anObject)[-1].retained == UINT_MAX - 1)

[NSException raise: NSInternalInconsistencyException

arrange: @"NSIncrementExtraRefCount() asked to increase excessively far"];

((struct obj_layout *)anObject)[-1].retained++; }

at the point when the variable "held" floods, it's increased.



- (void) discharge {

in the event that (NSDecrementExtraRefCountWasZero(self))

[self dealloc];



NSDecrementExtraRefCountWasZero(id anObject) {

in the event that (((struct obj_layout *)anObject)[-1].retained == 0) {

return YES;

} else {

((struct obj_layout *)anObject)[-1].retained- - ; restore NO;



"held" decremented.



- (void) dealloc {

NSDeallocateObject (self);


inline void NSDeallocateObject(id anObject) {

struct obj_layout *o = &((struct obj_layout *)anObject)[-1];



a chunck of memory desposed.


like "programmed variable" in C, which is discarded naturally when the execution leaves the degree.

autorelease implies when execution leaves a code hinder, the discharge strategy is approached the protest consequently.


- (id) autorelease {

[NSAutoreleasePool addObject:self];


ARC(Automatic Reference Counting)

With ARC, "id" and protest sort factors must have one of the accompanying proprietorship qualifiers:

_strong, _weak, __unsafeunretained, _autoreleasing

Possession is legitimately overseen by factor scope, as well as by assignments between factors, which are qualified with __strong.

Utilize __weak to maintain a strategic distance from roundabout references:

A __weak possession qualifier gives a feeble reference. A feeble reference does not have responsibility for protest.


id __strong obj0 = [[NSObject alloc] init];

id __weak obj1 = obj0;


* variable obj1 has a frail reference of the made question */

_unsafe_unretained: don't utilize it unless you need to help before iOS5, it may leave a dangling pointer. utilize _weak.

Be that as it may, __weak is solely for pointers that ought to be focused, not for primitive sorts.

_unsafe_unretained is helpful for circumstances where you have a question pointer in a circumstance where ARC doesn't have enough control to securely oversee memory, as within a standard C struct. That is one reason there's the "no protest pointers in C struct" restriction, unless you brighten that point with _unsafe_unretained to tell the compiler you what you're doing. It's likewise used to break solid reference cycles (a.k.a. hold cycles) with hinders under some particular conditions.

Be that as it may, for 99.9% of every day work, feeble works incredible and you can disregard __unsafe_unretained.



A Block is created from the Block strict beginning with "^". Also, the Block is appointed to the variable "blk". Obviously you can appoint the incentive to different factors of the Block sort.

Pieces are Anonymous capacities together with automatic(local)variables.

Assertions (source)

As a nearby factor:

returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};

As a property:

@property (nonatomic, duplicate) returnType (^blockName)(parameterTypes);

As a technique parameter:

(void)someMethodThatTakesABlock:(returnType (^)(parameterTypes))blockName;

As a contention to a technique call:

[someObject someMethodThatTakesABlock: ^returnType (parameters) {...}];

As a typedef:

typedef returnType (^TypeName)(parameterTypes);

TypeName blockName = ^returnType(parameters) {...};


int (^blk1)(int) = blk;

int (^blk2)(int); blk2 = blk1;

/*Functions can take contentions of Block type.*/

void func(int (^blk)(int)) {

/*Also, capacities can restore a Block.*/

int (^func()(int)) {

return ^(int count){return tally + 1;};


Utilizing typedefs:


typedef int (^blk_t)(int);

Terrific Central Dispatch

Step by step instructions to utilize: characterize assignments you need to execute and add them to a suitable dispatch line.
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