Subclasses and target casting C

Question

Today I ran into some strange problem. I created a subclass of UIView and added only 1 method to the template code provided by xcode.

@interface FloatView : UIView { } - (void)floatTest:(CGFloat)x; @end - (void)floatTest:(CGFloat)x { NSLog(@"float was %f", x); } 

Then in my appDelegate I had code like this:

 UIView *floatView = [[FloatView alloc] init]; [floatView floatTest:10.0f]; 

Pretty simple, right? What does it print? I thought it would be something like "10.0000", but no, it prints "0.000000".

I struggled with this watch trying to figure out what I am doing wrong, and then I changed the code in the appDelegate app to

 FloatView *floatView = [[FloatView alloc] init]; [floatView floatTest:10.0f]; 

Only after that I printed the expected "10.0000". Why is this so? I declared FloatView as a subclass of UIView, should I not be able to assign a FloatView object to a UIView pointer without problems?

Despite the fact that the floatView was declared a pointer to a UIView, is it really a floatView and should be able to handle the floatTest message? Am I completely out of here?

Answer 1

In fact, polymorphism works as expected. If this did not work, nothing would be printed (0.0000 is printed in your example). The fact is that your instance actually responds to the testFloat:10.0f , since the compiler cannot statically see the method declaration (since the UIView class UIView not declare such a method), it assumes that your method takes ... as an argument and returns id .

When CGFloat is passed to a method that expects a variable number of arguments ( ... ), it advances to double . Thus, the receive method is passed with the double argument and considers it a float , and it does not print correctly.

You can check this behavior by changing the NSLog line to:

 NSLog(@"%f", *(double*)&x); 

When the compiler sends the message FloatView* , not UIView* , it can find the exact signature of the method. He can see that he really expects CGFloat and does not support the double argument. As a result, it works correctly.

Also, if UIView* contains the method declaration that CGFloat took, the compiler would CGFloat this method accordingly. To summarize, this is not a polymorphism problem; this is a missing signature issue.