问题
Why do Objective-c objects have to be dynamically allocated? Why do I have to make it a pointer to an object, unlike in C++ I can create them on stack? Thanks.
回答1:
the primary reason: not knowing how much stack size to reserve.
existing conventions and uses also make lifting the restriction quite difficult.
dynamic messaging does not matter in this case, as setting the right 'vtable' at initialization is trivial.
in c++, the size of a stack object is always known (and if it's wrong, you know what to expect). an objc alloc/init sequence can return any one of several types -- each with different sizes (it's essentially a factory), or nothing at all.
the size can also vary at runtime (e.g. you can add fields to a class via the runtime).
Update 1
i was curious about this, so i made a little test program as a proof of concept.
i was able to implement a simple stack based objc class hierarchy, which also implemented a good chunk of NSObject's interface -- of course, omitting the reference counting and dynamic allocation interfaces as it did not relate to the proof of concept. at any rate, my simple class hierarchy was not fully compatible with the NSObject class or protocol, so it isn't something that should be used where NSObject types are expected, for obvious reasons. therefore, it is possible (and not particularly difficult) to accomplish this, if you really wanted stack based objc objects.
you don't have to do anything different from c++ to reserve the stack space. the stack size to reserve is still a restriction in some areas (consider factory methods, class clusters, etc.).
there are also a few runtime functionalities which will not work by default. the best example here is the ability to add ivars at runtime. you could in fact accomodate this functionality, if you needed it. i didn't bother with that exercise.
naturally, the base interface could take several deviations - one deviation that i made for fun was adding the ability to exchange the implementations (type) of a living object.
have fun
Update 2
as it turns out, GCC accepts the proof of concept i wrote. unfortunately, this has been banned in clang due to problems/dangers which can be encountered in reserving the correct size (considering the dynamic features of the language...). example: clang forbids sizeof(NSObject). oh well.
回答2:
Objective-c is a dynamic language, which means everything about it can change at runtime. The object's class object is only created when it is loaded from the executable, and it can be changed by categories. Also, the runtime can instance variables for properties. Since so much about the object can change after compilation, it cannot be created until it is used.
回答3:
This is because of the way Objective-C uses dynamic or late binding. Unlike C++ where you'll always have the choice between calling a function of a class by it's object or via a pointer of the same class or even of a superclass. In the latter case polymorphism is required.
However, in Objective-C there exists always the ability to determine the correct function at runtime. The difference is, that for example in C++ the compiler has to ensure, that the used function exists, whereas in Objective-C the compiler doesn't really care, the runtime system decides only.
来源：https://stackoverflow.com/questions/5844284/why-do-objective-c-objects-have-to-be-dynamically-allocated