Re: Covariant return types doesn't work (with g++ 4.1.2)
Alf P. Steinbach wrote:
* Thomas J. Gritzan:
mr.xiaofan.li@gmail.com wrote:
class virt_base
{
[...]
virtual virt_base* cut()
{
return new virt_base();
}
[...]
};
class virt_derived
: public virt_base
{
[...]
virtual virt_derived* cut()
{
return new virt_derived();
}
[...]
};
The covariant return type lets you override a function with a more
strict function, i.e. a function returning a derived type. However,
type checking is done at compile time, so your compiler has to know,
that the object you are calling cut() on is a virt_derived.
int main()
{
virt_base* my_virt_derived = new virt_derived();
Change this to
virt_derived* my_virt_derived = new virt_derived();
That would make the code compile, but would defeat the pupose of the code.
That would be with your code change:
virt_base* my_virt_derived = new virt_derived();
virt_base* new_virt_derived = my_virt_derived->cut();
No need for covariant return types, since both pointers are virt_base* and
virt_base::cut returns virt_base*.
Since we want to show how to use covariant return types, we have to declare
both pointers as virt_derived*.
I guess one of us needs a cup of coffee again :-)
--
Thomas
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