comp.lang.c++

On Sun, 10 Feb 2008 19:46:31 +0100, Alf P. Steinbach wrote:

> * Leon:
>> On Sun, 10 Feb 2008 10:01:47 -0800, mr.xiaofan.li wrote:
>>
>>> Hi,
>>>
>>> I have been totally confused about the covariant return type feature
>>> in C++. Below is an example code I wrote that doesn't compile with g++
>>> 4.1.2 (on Fedora 8)
>>>
>>> class virt_base
>>> {
>>> public:
>>> virt_base()
>>> {
>>> }
>>>
>>> virtual ~virt_base()
>>> {
>>>
>>> }
>>>
>>> virtual virt_base* cut()
>>> {
>>> return new virt_base();
>>> }
>>>
>>> void say_hi()
>>> {
>>> cout <<"hi!!! " <>>> }
>>> };
>>>
>>> class virt_derived
>>> : public virt_base
>>> {
>>> public:
>>> virt_derived()
>>> {
>>> }
>>>
>>> ~virt_derived()
>>> {
>>>
>>> }
>>>
>>> virtual virt_derived* cut()
>>> {
>>> return new virt_derived();
>>> }
>>>
>>> void say_hi()
>>> {
>>> cout <<"HI!!!! " <>>> }
>>> };
>>>
>>> int main()
>>> {
>>> virt_base* my_virt_derived = new virt_derived(); virt_derived*
>>> new_virt_derived = my_virt_derived->cut(); // g++
>>> complains here: invalid
>>> //
>>> conversion from 'virt_base*' to
>>> //
>>> 'virt_derived*'
>>> new_virt_derived->say_hi();
>>> }
>>>
>>> ------------
>>> It is very weird. Can somebody enlighten me what went wrong here?
>>> Thanks in advance.
>>
>> try changing your main() to this:
>>
>> int main()
>> {
>> virt_base* my_virt_derived = new virt_derived(); virt_derived*
>> new_virt_derived = (virt_derived*)my_virt_derived-
>>> cut();
>
> To the OP:
>
> Don't follow that advice.
>
> Casts can often make code compile, at the expense of most often being
> incorrect, and hiding future errors.
>
> The cast says "I know what I'm doing", when that is absolutely not so.
>
> In short the above is extremely bad advice: it is advicing you to lie
> and bluff your way.
>
> Don't pay any heed to folks who think a cast (and especially a C style
> cast) is a solution.
>
> They simply don't know anything about what they're talking about.
>
>
>> }
>
>
> Cheers, & hth.,
>
> - Alf

You think casts are useless? Then I want you to read some integers from a
fstream, since fstream's read method returns chars only. In some cases
you are right, and is a cast stupid. But maybe it works for this case.
And if i say something wrong, please explain me my mistake, and don't
respond so unkind. Or do you want to say you never make mistakes?

Grz, Leon