Checking array indexes when using literal subscripting

Question

As Apple suggests, we need to check the index of an NSArray before calling objectAtIndex:. To do this, we can use a category or method swizzling.

For this subscript syntax,

index = 1;
id person = pArray[index];

is there a way to implement that? Of course, I can check index beforehand.

You want to check this for every array without opt out? That will add a lot of overhead and is definitely not need in most cases (for example when iterating over a range, or when the count is validated once after the array is received from the network or similar) — JustSid
– JustSid, Commented Feb 12, 2015 at 4:52

JustSid · Accepted Answer · 2015-02-12 05:57:16Z

1

The compiler simply generates the fancy new syntax sugar into a call to objectAtIndexedSubscript:, so that would be the method you want to swizzle. As mentioned in my comment though, that will add a lot of overhead that you can't easily get out of after enforcing it.

Edit: One more thought on the overhead, NSArray already does that check internally and throws an exception when it comes out of bound. Maybe wrapping some blocks of code in @try/@catch blocks where it is sensible makes more sense for you?

edited Feb 12, 2015 at 5:57

answered Feb 12, 2015 at 4:53

JustSid

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2 Comments

scorpiozj Over a year ago

ah, objectAtIndexedSubscript is what I want~

scorpiozj Over a year ago

no need to care about why the exception happens and hence abandon using @try/catch. If index is out of range, it gets a nil object which will be safe even sent a not found selector.

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