On 9 Jun 2011, at 00:10, Nilay Vaish wrote:
> On Wed, 8 Jun 2011, Jack Harvard wrote:
>
>>
>> On 8 Jun 2011, at 23:28, Nilay Vaish wrote:
>>
>>> On Wed, 8 Jun 2011, Jack Harvard wrote:
>>>
>>>>
>>>>
>>>> On 8 Jun 2011, at 19:09, Nilay Vaish wrote:
>>>>
>>>>> On Wed, 8 Jun 2011, Jack Harvard wrote:
>>>>>
>>>>>> When you declare your function private, you can't use
>>>>>> instance.function() to access it. Is it generating a compile time error?
>>>>>>
>>>>>> On 8 Jun 2011, at 00:31, Nilay Vaish wrote:
>>>>>>
>>>>>>> Consider the following class declarations --
>>>>>>>
>>>>>>> class A
>>>>>>> {
>>>>>>> public:
>>>>>>> virtual void function() = 0;
>>>>>>> };
>>>>>>>
>>>>>>> class B : public A
>>>>>>> {
>>>>>>> private:
>>>>>>> void function();
>>>>>>> }
>>>>>>>
>>>>>>> int main()
>>>>>>> {
>>>>>>> B b;
>>>>>>> b.function();
>>>>>>> }
>>>>>>>
>>>>>>> Will this code compile correctly?
>>>>>>>
>>>>>>> --
>>>>>>> Nilay
>>>>>
>>>>> I should say that my example program was not what I intended it to be.
>>>>> The main function should look like --
>>>>>
>>>>> int main()
>>>>> {
>>>>> B* b = new B();
>>>>> A* a = b;
>>>>> a->function();
>>>>> return 0;
>>>>> }
>>>>>
>>>>> Now what would happen?
>>>>
>>>> This compiles. However, if you do b->function(), you would get the same
>>>> error as your last example, due to the same reason.
>>>>
>>>
>>> It compiles and executes fine. What surprises me is that even though
>>> function() is private for class B, still it gets invoked using the pointer
>>> from class A. I was not aware of this before.
>>
>> Overriding and access visibility is orthogonal, you use class A pointer to
>> access its public function.
>
> I won't term this is a overriding, the function that will be called would be
> the one defined in class B, as 'function()' is a virtual member of class A.
> But then, 'function()' is private to class B, so I would expect some error to
> occur. I think the reason is that visibility is tested only at compile time
> and never at run time.
It's still overriding for the function() defined in B which is overriding the
function defined in base class A (whether it's defined as virtual or pure
virtual). In C++ it's allowed to override with a private member. That means you
can only call it via a pointer or reference to the base. This is occasionally
useful (eg if the base is a private one), but it isn't very common in my
experience.
(A long, long time ago there were rules in C++ like those in Java to
prevent derived classes reducing the visibility of members. They were
abandoned because they got in the way of reasonable code.)
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