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The question is from a comment I just added to the answer to this question, but it shouldn't be a duplicate.

The answer from @Bavarious to that question makes sense to me, but I am still confused why the runtime can not bind the method to the right object even the object is an id? to my understanding, dynamic binding or dynamic typing is that the compiler has no way of knowing what object behind an id, but the runtime is supposed to know that and choose the right object as the receiver of the message. but why the runtime can't do that?

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neevek
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  • Note: You can ask the compiler to emit warnings (in some scenarios), by enabling `-Wstrict-selector-match`. Even better, message type-qualified variables and parameters, rather than bare `id`s. – justin Feb 27 '12 at 05:52

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Short answer: C.

Expansion: When the compiler generates a call to a method, it is really just generating a call to a C function -- to objc_msgSend() or variant therein -- that dynamically binds and dispatches the method.

Because the C ABI runs "at the metal" (pretty close), the way that arguments and return values are passed can change dramatically based on the type. For example, the compiler can pass anything up to 32 bits in size in a register (or 64 bits in some architectures), but anything larger will be on the stack itself.

Because of this, the compiler can't necessarily emit a generic bit of assembly to call objc_msgSend() -- to dispatch the method call -- through (id) as the different signatures -- the different argument types to the method -- may require different code generation.

Now, technically, the compiler could generate the same code for many cases, but it chooses to take a conservative policy and complain. In particular, the assumption is that if you have 2 conflicting typed declarations, there may be others and reminding you that having a method of the same name with different argument types is extremely strongly discouraged.

bbum
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  • Thanks you for the swift reply. Now I can see the point that with conflicting typed declarations, the compiler may generate the same code, which is ambiguous to the runtime. – neevek Feb 27 '12 at 06:14
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    Happy to help! If you want to learn a lot more than is necessary about objc_msgSend() and dispatch, I wrote a series of weblog articles about it: http://www.friday.com/bbum/2009/12/18/objc_msgsend-part-1-the-road-map/ – bbum Feb 27 '12 at 06:52