Re: Covariant return types doesn't work (with g++ 4.1.2)
* mr.xiaofan.li@gmail.com:
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
Note that the term "virtual base (class)" has a special meaning in C++.
Your class "virt_base" is not used as a virtual base class.
{
public:
virt_base()
{
}
virtual ~virt_base()
{
}
virtual virt_base* cut()
{
return new virt_base();
}
void say_hi()
{
cout <<"hi!!! " <<endl;
}
};
class virt_derived
: public virt_base
{
public:
virt_derived()
{
}
~virt_derived()
{
}
virtual virt_derived* cut()
{
return new virt_derived();
}
void say_hi()
{
cout <<"HI!!!! " <<endl;
}
};
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*'
Please be a little more conscientious with your quoting. This code
can't be copied and compiled. It must be manually reformatted.
The problem is simply that virt_base::cut() returns a virt_base*.
So you're initializing a virt_derived* pointer with a virt_base* pointer.
Declare new_virt_derived as
virt_base* new_virt_derived = ...
In addition, to get the result I think you expect, you'll have to
declare say_hi as virtual in virt_base.
new_virt_derived->say_hi();
}
Cheers, & hth.,
- Alf
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