保留Objective-c（再次但更精确）

Question

I post this question again but with more precision this time,

first, I have this function who return a string, there is some error about memory management or this fonction is ok?

-(NSString *) motAvecCle:(NSString *) cle
{
    NSString *motRetour;

    motRetour = @"";

    cle = [FonctionUtile concatener:[[tableauConfig singletonTableauConfig] langueAffichage] chaine2:@"_" chaine3:[FonctionUtile trim:[cle uppercaseString]] chaine4:@""];

    motRetour =  [FonctionUtile trim:[dictionnaireLangue objectForKey:cle]];

    if (motRetour == nil) {
        motRetour = @"Erreur";
    } 

    return motRetour;
}

and when I call this fonction,

NSString *myString = @"";
myString = [self motAvecCle:@"fr"]; // I must do this?
myString = [[self motAvecCle:@"en"]retain]; //or do this?

thx again...

Answer 1

The method motAvecCle: returns an object you do not own. Therefore, at some point it is going to disappear. Whether you care or not depends on where myString is defined. If it's in the same scope:

-(void) foo
{
    NSString *myString = [self motAvecCle:@"fr"];
    // do some stuff
}

you do not want to retain it (except in one circumstance) because the reference will disappear when foo exits which means if you had retained it you'd need to release it again first.

The one circumstance for retaining is if you modify the object you got the string from ie self in this case. That might cause the string to go away (although probably not in your specific example).

If myString is an instance variable of the object, you do want to retain it because otherwise it will disappear (possibly) the next time the auto release pool is drained. However, before assigning the instance variable, you must be sure to release the old value of the instance variable, unless it's actually the same string you are assigning ie you need to do something like this:

-(void) foo
{
    NSString *tmp = [[self motAvecCle:@"fr"] retain]; // it's a string, technically you should copy, not retain
    [myString release];
    myString = tmp;
    // do some stuff
}

Since you'd have to do that every time you want to assign the ivar, it's normal to create an accessor eg

-(void) setMyString: (NSString*) newValue
{
    NSString* tmp = [newValue copy];
    [myString release];
    myString = tmp;
}

-(void) foo
{
    [self setMyString: [self motAvecCle:@"fr"]];
    // do some stuff
}

If you use properties, you can use @synthesize to create the accessors.