Results for performSelector: differ from standard dot notation call?

Question

I was very puzzled by some unexpected compile errors in my code recently. I did some troubleshooting, and I came up with this bit of test code:

1 float performSelectorResult, messageResult;
2 SEL selector = @selector(smallContentCellMargin);
3
4 NSLog (@"selector %s result %f", selector,  [defaults performSelector:selector]);
5 NSLog (@"message result is %f", defaults.smallContentCellMargin);
6
7 performSelectorResult = [defaults performSelector:selector];
8 messageResult = defaults.smallContentCellMargin;

If I run the code as-is, I get a compile error on line 7: "incompatible types in assignment". However, if I comment out line 7 the code build and runs; the results in NSLog are:

2009-07-21 18:31:44.823 ICE[24347:20b] selector smallContentCellMargin result 0.000000
2009-07-21 18:31:44.830 ICE[24347:20b] message result is 7.000000

I've used very similar code to retrieve a UIFont, but never a float. Is there something I don't know, or is this a bug in Objective-C? I'm very confused.

Answer 1

First, this you are treating a selector as a string when logging with NSLog. Don't do that - a SEL is not a string. Use the %@ format and NSStringFromSelector(aSelector).

For the issue at hand, -performSelector: is defined to work with any selector that has an object return type. Due to the ABI, you can get away with using this for int return types up to the size of a pointer (depending upon runtime implementation details), but you generally cannot get away with it for float return values, as you've noticed.

For non-object return values, you should use NSInvocation, or -valueForKey:. In the latter case, you'll get a float boxed as an NSNumber.