Question

In Apple’s The Objective-C Programming Language: Defining a Class the section named “Redefining self” recommends that that class methods allocate and return instances use ‘self’ only to allocate an instance and then refer only to that instance. Thus, I have a number of subclasses, that have class methods along the lines of:

+ (id)scrollViewWithFrame: (NSRect)rectFrame
{
    id newInstance = [[[self alloc] initWithFrame:rectFrame] autorelease];
    [newInstance setHasHorizontalScroller: YES];
    [newInstance setHasVerticalScroller: YES];
    [newInstance setBorderType: NSNoBorder];
    [newInstance setAutoresizingMask: (NSViewWidthSizable
                | NSViewHeightSizable)];

    return newInstance;
}

The above is, of course, a subclass of NSScrollView. Unfortunately, with Xcode 3.x all these NSView subclasses now raise warnings: “Warning: Multiple methods named ‘-setAutoresizingMask’ found”. I believe it has something to do with GCC 4.2, which uses the Xcode default settings.

The warning is correct, of course, since NSView and its various subclasses all implement setAutoresizingMask, but it is also unnecessary. Since they’re only warnings, I ignore them but there is a risk that in between the thirty or so unnecessary ones, a really useful warning lurks which I simply don’t see. So, what to do? I do want to adhere to good coding practices and I want to build warning-free apps — how can I do both?

Answer 1

That warning about “multiple methods found” means that more than one class implements setAutoresizingMask: and the compiler can’t tell which one this is because the variable is typed id. The compiler is warning you that it’s possible (since the class of the object is unknown) that you might get the wrong one. Try typing newInstance either as an NSScrollView* or as your class and the warning will go away. You can still return id, since it’s equivalent to all object-pointer types. It’s just the variable you’re sending messages to that the compiler wants to be statically typed.