Casting and Obj-C

[Jonnebob]

Is casting a bad thing? What are peoples thoughts on the matter? Should it be avoided where possible? or a necessary evil?

The reason I ask is that i'm following the Stanford lectures and working my way through the assignments and on the first one they ask you to make an NSDictionary and only print out the values from entries into the dictionary which have a key with a certain prefix.

Thats fine I thought as I wrote my first verision of the for loop:

for (NSDictionary *entry in bookmarkDict)
		
	{
		if ([entry hasPrefix:@"Stanford"]) 
		{
			NSLog(@"Key: '%@' URL: '%@'", entry, [bookmarkDict objectForKey:entry]);
		}
		
	}

Without using the Cast I get an warning: "warning: 'NSDictionary' may not respond to '-hasPrefix:'"

Whats the problem you may be thinking? A Simple cast and the warning dissappears! Well me being the perfectionist I would quite like my finished solution (and for future reference too) to have no cast, if possible.

So, have I missed a method name somewhere in the documentation or is this a case where a cast is the only solution?

[ftm]

Casting is the right way to go as long as you know you are casting to the right object type.

[smasher]

In this case you already have one cast, but I think it's casting to the wrong class. If bookmarkDict is full of strings, then your loop should be:

for (NSString *entry in bookmarkDict)

Casting is the way to go in situations like this, when dealing with arrays and dictionaries. The compiler has no idea what type of object is in an array or dictionary, so you need to help it out.

There are other situations where casting would be wrong. I can create and NSString and cast it as a UIView, but that won't make it respond to the right messages for a UIView. It doesn't actually do any "conversion" on the object.

[FlyingDiver]

Change your code to this to get rid of the warning:

for (id entry in bookmarkDict)
		
	{
		if ([entry hasPrefix:@"Stanford"]) 
		{
			NSLog(@"Key: '%@' URL: '%@'", entry, [bookmarkDict objectForKey:entry]);
		}
		
	}

The problem you have is that the enumerator for iterating through the dictionary is not a (NSDictionary *). It's a generic pointer (id) because you don't actually know what the dictionary entry type is.

joe

[Jonnebob]

Thanks for all your replies.

I think part of my problem was that I was coming from java/php background I wasn't used to defining the type of element in the enumerator.

FlyingDiver: changing to id, causes two different warnings.

"invalid reciever type 'id*'"

"passing argument 1 of 'objectForKey:' from incompatible pointer type".

Using NSString in the enumerator doesnt produce any warnings. I think this must be because the the NSDictionary enumerator is looping through the keys which it knows are strings.

[FlyingDiver]

Originally Posted by Jonnebob

Thanks for all your replies.

I think part of my problem was that I was coming from java/php background I wasn't used to defining the type of element in the enumerator.

FlyingDiver: changing to id, causes two different warnings.

"invalid reciever type 'id*'"

"passing argument 1 of 'objectForKey:' from incompatible pointer type".

Using NSString in the enumerator doesnt produce any warnings. I think this must be because the the NSDictionary enumerator is looping through the keys which it knows are strings.

Read the code above carefully. It's "id", not "id *". "id" is already a pointer, you don't add the "*" to it. Using "id" is the method in the Fast Enumerator documentation.

joe

[Jonnebob]

Ah yes, I see. My bad.

Thanks for all your help.

[jsd]

Originally Posted by Jonnebob

Using NSString in the enumerator doesnt produce any warnings. I think this must be because the the NSDictionary enumerator is looping through the keys which it knows are strings.

I know you already have a working answer but I want to mention here that writing

for (NSString *entry in bookmarkDict)

absolutely does NOT only filter out NSString entries! It simply tells the compiler that you want to treat each item as an NSString. If the item in question actually isn't an NSString, horrible bad things can happen...