[jonhohle]

> Without the method types, the compiler will print a warning, infer the wrong ABI and generate the wrong code.

Did you even try my example? No compiler warnings are generated (nor should the be). What do you mean by defined method types? It is simply looking for any selector which matches on any class because there is not enough statically available information to know any different. Messages are always passed dynamically.

Are we talking about Objective-C? Are you familiar with NSInvocation? Or performSelector:, performSelector:withObject:, performSelector:withObject:withObject:? Or NSNotificationCenter's addObserver:selector:name:object:? This is all done at runtime. No special type information is available to the compiler when using these. Objective-C messagse are always sent dynamically, so the only ABI concerns are how the stack is prepared, and not the interface of the class of an object. You can define methods and swap them out at runtime, this feature would be useless if everything had to be known at compile time.

ARC needs to know that the types of Objective-C objects, id still works fine, beyond that it needs to know no other type information from what I can tell.

It seems we are talking past each other. Objective-C is not like C++, though. All methods are virtual, always. The runtime goes through great pains to make that efficient and still allow complete dynamism. This is orthogonal from ARC.

[nupark2]

> Did you even try my example? No compiler warnings are generated (nor should the be).

Only because it managed to match on a defined method type. If a class declaration hadn't been found at compile time with the given declared method, it would have issued a warning.

If the match was ambiguous and the types incorrect, it would have emitted incorrect code, and possibly a warning (or always, with -Wstrict-selector-match).

> What do you mean by defined method types? It is simply looking for any selector which matches on any class because there is not enough statically available information to know any different.

By 'defined method types', I mean methods declared on visible classes that match the given selector.

If it matches on the wrong one, the wrong dispatch function and/or the wrong function call epilogue will be emitted.

Method calls are ABSOLUTELY NOT ABI identical for all possible types. I can't possibly emphasize this enough.

For example:

  - (void) performWithObject: (NSObject *) object;

  - (void) performWithObject: (NSObject *) firstObj, ...;

The instructions emitted for a vararg dispatch ARE NOT the same as the non-vararg dispatch on all platforms, and incorrect method selection will result in undefined behavior on dispatch.

> Are you familiar with NSInvocation? Or performSelector:, performSelector:withObject:, performSelector:withObject:withObject:? Or NSNotificationCenter's addObserver:selector:name:object:? This is all done at runtime.

> No special type information is available to the compiler when using these.

Yes, it is. Methods have associated type encodings that describe the return and argument types, and that's used to perform runtime dispatch with NSInvocation. This is why NSInvocation is so slow -- similar to libffi, it must evaluate the types and construct the call frame at runtime. It does this by evaluating the type data associated with method implementations by the compiler.

Methods such as performSelector rely on specific type conventions (such as void return, optional single object argument) and will fail if used with targets that do not match the expected convention.

[jonhohle]

You're right. I hadn't realized the compiler not only ensures the selector exists, but does type C-style type checking on dynamic calls as well. I was surprised to see that two messages with the same selector but different parameter types required a type cast to use.

Of course IMPs aren't identical if they take different parameters. This doesn't affect interchanging Objective-C types though. Yes, the arity and order are important, but the compiler doesn't enforce anything beyond that a pointer is passed for id types.

[nupark2]

Cheers to the peer comment regarding HN discourse. Unfortunately (?) I have more :)

> This doesn't affect interchanging Objective-C types though. Yes, the arity and order are important, but the compiler doesn't enforce anything beyond that a pointer is passed for id types.

This is true prior to ARC: all ObjC pointers are the same size, and hence ABI-compatible given equivalent arity/order. It's theoretically possible that a future ABI could be incompatible between two methods returning void vs pointer return value, but currently, all supported ABIs return pointer-sized values in a register.

However, with ARC, this changes. The type system has been effectively extended to denote the required referencing behavior for calling code. This means that for a given arity/order, you must also have equivalent referencing attributes.

[adriand]

I still find it disconcerting on HN when an argument ends with someone saying, "you're right". It's like the normal rules of the Internet just don't apply here. It's one of my favourite things about this community. To you, personally: kudos for maintaining that spirit!