Re: NSDictionary trouble
Re: NSDictionary trouble
- Subject: Re: NSDictionary trouble
- From: mmalc Crawford <email@hidden>
- Date: Tue, 19 Jan 2010 21:15:50 -0800
On Jan 19, 2010, at 8:47 pm, Jeff Laing wrote:
> I wrote:
>>> No, you should just *retain* the result of dictionaryWithCapacity.
>
> mmalc wrote:
>> No, you shouldn't.
> With all due respect, why not?
>
Because it's difficult to imagine a common situation in which your advice will be valid, for example:
NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithCapacity:x];
// Add to the dictionary.
myInstanceVariable = [dict retain];
Wrong, because you're setting an instance variable directly instead of using an accessor.
NSMutableDictionary *dict = [[NSMutableDictionary dictionaryWithCapacity:x] retain];
// Use the dictionary.
[dict release];
Retaining the dictionary is pointless.
> Apple gives us all those convenience methods but you say we shouldn't use them?
>
Please don't misrepresent what I wrote. I didn't say that. There are situations in which the convenience API is useful and not discouraged. In particular, for example, if you're creating a dictionary as a return value from a method, then it's perfectly reasonable to use a convenience method:
- (NSDictionary *)dictionaryRepresentation {
NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithCapacity:5];
// Use the dictionary.
return dict;
}
(Also if you're using garbage collection, any of the convenience methods are perfectly reasonable.)
In most other cases, however, Jens and Ken have already addressed the principal issue: performance. You don't want to needlessly prolong the lifetime of objects.
>> If you want to assign the dictionary to an instance variable, you
>> should use an accessor method. In this case, it's still better to use
>> alloc/init then release the dictionary when you've finished with it.
> That's seems a really aberrant case, where you are creating an empty dictionary with a nominated capacity. I can't visualize a case where you would do such a thing, outside of the initialiser.
>
This pattern is shown commonly in iPhone sample code:
NSMutableDictionary *aDictionary = [[NSMutableDictionary alloc] initWithCapacity:x]
// Add to the dictionary.
self.dictionary = aDictionary;
[aDictionary release];
> There are cases like the NSFileManager where you need to pass in a dictionary of options that describe the directory being created (for example) - should I really be creating temporary variables for those, or using the convenience methods (dictionaryWithObject:forKey:)?
>
The same applies as above.
Following best practices, you should use alloc/init+release. How important this is to adhere to depends on how performance-sensitive your code and platform are...
To reiterate Jens' succinct formula:
Convenience ≠ performance.
mmalc
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