您的位置：首页 > 其它

Memory Management Policy

2014-09-01 20:39 351 查看

Thebasicmodelusedformemorymanagementinareference-countedenvironmentisprovidedbyacombinationofmethodsdefinedinthe

NSObject

protocoland
astandardmethodnamingconvention.The

NSObject

classalsodefinesamethod,

dealloc

,
thatisinvokedautomaticallywhenanobjectisdeallocated.ThisarticledescribesallthebasicrulesyouneedtoknowtomanagememorycorrectlyinaCocoaprogram,andprovidessomeexamplesofcorrectusage.

BasicMemoryManagementRules

Thememorymanagementmodelisbasedonobjectownership.Anyobjectmayhaveoneormoreowners.Aslongasanobjecthasatleastoneowner,itcontinuestoexist.Ifanobjecthasnoowners,theruntimesystem
destroysitautomatically.Tomakesureitisclearwhenyouownanobjectandwhenyoudonot,Cocoasetsthefollowingpolicy:

Youownanyobjectyoucreate
Youcreateanobjectusingamethodwhosenamebeginswith“alloc”,“new”,“copy”,or“mutableCopy”(forexample,

alloc

newObject

,
or

mutableCopy

).

Youcantakeownershipofanobjectusingretain
Areceivedobjectisnormallyguaranteedtoremainvalidwithinthemethoditwasreceivedin,andthatmethodmayalsosafelyreturntheobjecttoitsinvoker.Youuse

retain

in
twosituations:(1)Intheimplementationofanaccessormethodoran

init

method,totakeownershipofanobjectyouwanttostoreasapropertyvalue;and(2)Toprevent
anobjectfrombeinginvalidatedasaside-effectofsomeotheroperation(asexplainedin“Avoid
CausingDeallocationofObjectsYou’reUsing”).

Whenyounolongerneedit,youmustrelinquishownershipofanobjectyouown
Yourelinquishownershipofanobjectbysendingita

release

message
oran

autorelease

message.
InCocoaterminology,relinquishingownershipofanobjectisthereforetypicallyreferredtoas“releasing”anobject.

Youmustnotrelinquishownershipofanobjectyoudonotown
Thisisjustcorollaryofthepreviouspolicyrules,statedexplicitly.

ASimpleExample

Toillustratethepolicy,considerthefollowingcodefragment:

Person*aPerson=[[Personalloc]init];

//...

NSString*name=aPerson.fullName;

//...

[aPersonrelease];

ThePersonobjectiscreatedusingthe

alloc

method,soitissubsequentlysenta

release

message
whenitisnolongerneeded.Theperson’snameisnotretrievedusinganyoftheowningmethods,soitisnotsenta

release

message.Notice,though,thattheexampleuses

release

rather
than

autorelease

UseautoreleasetoSendaDeferredrelease

Youuse

autorelease

whenyouneedtosendadeferred

release

message—typically
whenreturninganobjectfromamethod.Forexample,youcouldimplementthe

fullName

methodlikethis:

-(NSString*)fullName{

NSString*string=[[[NSStringalloc]initWithFormat:@"%@%@",

self.firstName,self.lastName]autorelease];

returnstring;

Youownthestringreturnedby

alloc

.Toabidebythememorymanagementrules,youmustrelinquishownershipofthestringbefore
youlosethereferencetoit.Ifyouuse

release

,however,thestringwillbedeallocatedbeforeitisreturned(andthemethodwouldreturnaninvalidobject).Using

autorelease

,
yousignifythatyouwanttorelinquishownership,butyouallowthecallerofthemethodtousethereturnedstringbeforeitisdeallocated.
Youcouldalsoimplementthe

fullName

methodlikethis:

-(NSString*)fullName{

NSString*string=[NSStringstringWithFormat:@"%@%@",

self.firstName,self.lastName];

returnstring;

Followingthebasicrules,youdon’townthestringreturnedby

stringWithFormat:

,soyoucansafelyreturnthestringfrom
themethod.
Bywayofcontrast,thefollowingimplementationiswrong:

-(NSString*)fullName{

NSString*string=[[NSStringalloc]initWithFormat:@"%@%@",

self.firstName,self.lastName];

returnstring;

Accordingtothenamingconvention,thereisnothingtodenotethatthecallerofthe

fullName

methodownsthereturnedstring.
Thecallerthereforehasnoreasontoreleasethereturnedstring,anditwillthusbeleaked.

YouDon’tOwnObjectsReturnedbyReference

SomemethodsinCocoaspecifythatanobjectisreturnedbyreference(thatis,theytakeanargumentoftype

ClassName**

id*

).
Acommonpatternistousean

NSError

objectthatcontainsinformationaboutanerrorifoneoccurs,asillustratedby

initWithContentsOfURL:options:error:

(

NSData

)
and

initWithContentsOfFile:encoding:error:

(

NSString

).
Inthesecases,thesamerulesapplyashavealreadybeendescribed.Whenyouinvokeanyofthesemethods,youdonotcreatethe

NSError

object,
soyoudonotownit.Thereisthereforenoneedtoreleaseit,asillustratedinthisexample:

NSString*fileName=<#Getafilename#>;

NSError*error;

NSString*string=[[NSStringalloc]initWithContentsOfFile:fileName

encoding:NSUTF8StringEncodingerror:&error];

if(string==nil){

//Dealwitherror...

//...

[stringrelease];

ImplementdealloctoRelinquishOwnershipofObjects

The

NSObject

classdefinesamethod,

dealloc

,
thatisinvokedautomaticallywhenanobjecthasnoownersanditsmemoryisreclaimed—inCocoaterminologyitis“freed”or“deallocated.”.Theroleofthe

dealloc

method
istofreetheobject'sownmemory,andtodisposeofanyresourcesitholds,includingownershipofanyobjectinstancevariables.
Thefollowingexampleillustrateshowyoumightimplementa

dealloc

methodforaPersonclass:

@interfacePerson:NSObject

@property(retain)NSString*firstName;

@property(retain)NSString*lastName;

@property(assign,readonly)NSString*fullName;

@end

@implementationPerson

//...

-(void)dealloc

[_firstNamerelease];

[_lastNamerelease];

[superdealloc];

@end

Important:Neverinvokeanotherobject’s

dealloc

methoddirectly.
Youmustinvokethesuperclass’simplementationattheendofyourimplementation.
Youshouldnottiemanagementofsystemresourcestoobjectlifetimes;see“Don’t
UsedealloctoManageScarceResources.”
Whenanapplicationterminates,objectsmaynotbesenta

dealloc

message.Becausetheprocess’smemoryisautomaticallycleared
onexit,itismoreefficientsimplytoallowtheoperatingsystemtocleanupresourcesthantoinvokeallthememorymanagementmethods.

CoreFoundationUsesSimilarbutDifferentRules

TherearesimilarmemorymanagementrulesforCoreFoundationobjects(seeMemory
ManagementProgrammingGuideforCoreFoundation).ThenamingconventionsforCocoaandCoreFoundation,however,aredifferent.Inparticular,CoreFoundation’sCreateRule(see“The
CreateRule”inMemory
ManagementProgrammingGuideforCoreFoundation)doesnotapplytomethodsthatreturnObjective-Cobjects.Forexample,inthefollowingcodefragment,youarenotresponsibleforrelinquishingownershipof

myInstance

内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理

标签：

相关文章推荐

新的分享

章节导航