objective c - How do I return a struct value from a runtime-defined class method under ARC?

Question

I have a class method returning a CGSize and I'd like to call it via the Objective-C runtime functions because I'm given the class and method names as string values.

I'm compiling with ARC flags in XCode 4.2.

Method signature:

+(CGSize)contentSize:(NSString *)text;

The first thing I tried was to invoke it with objc_msgSend like this:

Class clazz = NSClassFromString(@"someClassName);
SEL method = NSSelectorFromString(@"contentSize:");

id result = objc_msgSend(clazz, method, text);

This crashed with "EXC_BAD_ACCESS" and without a stack trace. I used this first because the documentation for objc_msgSend says,

When it encounters a method call, the compiler generates a call to one of the functions objc_msgSend, objc_msgSend_stret, objc_msgSendSuper, or objc_msgSendSuper_stret. [...] Methods that have data structures as return values are sent using objc_msgSendSuper_stret and objc_msgSend_stret.

Next, I used objc_msgSend_stret like this:

Class clazz = NSClassFromString(@"someClassName);
SEL method = NSSelectorFromString(@"contentSize:");

CGSize size = CGSizeMake(0, 0);
objc_msgSend_stret(&size, clazz, method, text);

Using the above signature gives me the following two compiler errors and two warnings:

error: Automatic Reference Counting Issue: Implicit conversion of a non-Objective-C pointer type 'CGSize *' (aka 'struct CGSize *') to 'id' is disallowed with ARC

warning: Semantic Issue: Incompatible pointer types passing 'CGSize *' (aka 'struct CGSize *') to parameter of type 'id'

error: Automatic Reference Counting Issue: Implicit conversion of an Objective-C pointer to 'SEL' is disallowed with ARC

warning: Semantic Issue: Incompatible pointer types passing '__unsafe_unretained Class' to parameter of type 'SEL'

If I look at the declaration of the method, it is:

OBJC_EXPORT void objc_msgSend_stret(id self, SEL op, ...)
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

which is identical to objc_msgSend:

OBJC_EXPORT id objc_msgSend(id self, SEL op, ...)
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

This explains the compiler errors to me but what do I use at the run-time level to invoke a class and its static method at run-time and return a struct value?

Answer 1

You need to cast objc_msgSend_stret to the correct function pointer type. It's defined as void objc_msgSend_stret(id, SEL, ...), which is an inappropriate type to actually call. You'll want to use something like

CGSize size = ((CGSize(*)(id, SEL, NSString*))objc_msgSend_stret)(clazz, @selector(contentSize:), text);

Here we're just casting objc_msgSend_stret to a function of type (CGSize (*)(id, SEL, NSString*)), which is the actual type of the IMP that implements +contentSize:.

Note, we're also using @selector(contentSize:) because there's no reason to use NSSelectorFromString() for selectors known at compile-time.

Also note that casting the function pointer is required even for regular invocations of objc_msgSend(). Even if calling objc_msgSend() directly works in your particular case, it's relying on the assumption that the varargs method invocation ABI is the same as the ABI for calling a non-varargs method, which may not be true on all platforms.