Re: Covariant return types doesn't work (with g++ 4.1.2)
On Feb 10, 7:57 pm, "Alf P. Steinbach" <al...@start.no> wrote:
* Thomas J. Gritzan:
mr.xiaofan...@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.
What is the purpose of the code? (That's meant to be an honest
question. As far as I can tell, co-variant return types are a
solution looking for a problem---I've yet to find any reasonable
use for them.)
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