Question

So I am seemingly encountering some strange behavior when using NSString‘s -sizeWithFont family of method calls depending on whether or not I’m invoking it on the iPhone Simulator or an actual device.

Simply enough, when the receiver of the -sizeWithFont:constrainedToSize:lineBreakMode: method call is nil, the resulting CGSize passed back on the Simulator is {0, 0}. However, on the device, the return result is the same CGSize value that I specify in the method call as the constrainedToSize: parameter. See the following log statements:

Simulator:

someString: (null)
someStringSize: {0, 0}

Device:

someString: (null)
someStringSize: {185, 3.40282e+38}

The behavior on the Simulator is what I would expect. Not that this issue is difficult to circumvent, but 1) I’m a little confused why this family of functions would behave differently on the Simulator and an actual device, and 2) why does calling a method on a nil receiver return a particular, deterministic result?

Thanks for any pointers or insight you guys can provide!

EDIT: I suppose I should mention that I’m building against the 3.1 SDK.

Answer 1

Apple’s documentation on Sending messages to nil kind of explains this:

The value returned from a message to nil may also be valid:

• If the method returns an object, then a message sent to nil returns 0 (nil) …
• If the method returns any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0.
• If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the data structure. Other struct data types will not be filled with zeros.
• If the method returns anything other than the aforementioned value types the return value of a message sent to nil is undefined.

Although this document does not talk specifically about the iPhone (and it was written before the iPhone SDK was released), note the third and fourth bullet points: if the method returns a struct (such as CGSize), the behavior is platform-dependent. Only values that are returned in CPU registers are filled with zeros. I am not an expert on the ARM architecture of the iPhone, but it is likely that on the iPhone, the return value of sizeWithFont: is not returned in registers, as opposed to i386. In that case, the returned struct would be filled with zeros when running on the simulator while the value would be undefined when running on the iPhone.