comp.lang.c++

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();
}