Re: why Obj-C
Re: why Obj-C
- Subject: Re: why Obj-C
- From: Dan Crevier <email@hidden>
- Date: Mon, 08 Apr 2002 16:50:07 -0700
On 4/7/2002 11:18 PM, "Michael Gersten" <email@hidden> wrote:
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I'll probably show my C++ limit on this one, but...
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>
Andy Lee wrote:
>
>
>
> At 9:02 PM -0800 4/4/02, Dan Crevier wrote:
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>> On 4/4/2002 8:46 PM, "Andy Lee" <email@hidden> wrote:
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>>
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>>> * "self" is really "self", by which I mean the method that gets
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>>> invoked for an object depends on the object's actual class at
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>>> runtime, unlike C++ where the method invoked depends on the
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>>> variable's declared type.
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>>
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>> Maybe I'm just missing what you are saying here, but this is the basic
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>> meaning of polymorphism, and is the way C++ works too. For example:
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>>
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>> class A
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>> {
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>> public:
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>> virtual void foo();
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>> void bar() { this->foo(); }
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>> };
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>>
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>> class B
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>> {
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>> public:
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>> virtual void foo();
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>> };
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>>
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>> void f()
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>> {
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>> B *b = new b;
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>> A *bAsA = b;
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>>
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>> bAsA->foo(); // calls B::foo()
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>> bAsA->bar(); // calls B::foo()
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>> }
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>
First, does this compile? A *bAsA = B should provoke an error -- the two
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classes are incompatible.
I meant "class B : public A". Sorry about that.
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Second, that it works is an accident of the virtual table. In C++, calling a
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virtual function is really calling the n-th offset (known at compile time) of
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a table. Which table might not be known until run-time, but either its
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address, or a pointer to its address, or a pointer to a pointer to its address
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(a function argument) will be known at link time. (ObjC really requires
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looking it all up at run time, at least the first time). In class A, foo is
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the first function in that table. In class B, if B had another virtual
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function defined before foo, it would not.
With the inheritance fixed, this is no accident. It's the entire purpose of
a virtual function pointer/table.
Dan
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| | >Re: why Obj-C (From: Michael Gersten <email@hidden>) |