Re: Covariant return types doesn't work (with g++ 4.1.2)

From:

"Thomas J. Gritzan" <Phygon_ANTISPAM@gmx.de>

Newsgroups:

comp.lang.c++

Date:

Sun, 10 Feb 2008 19:49:10 +0100

Message-ID:

<fongv7$ftn$1@newsreader2.netcologne.de>

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();

    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();
}

--
Thomas
http://www.netmeister.org/news/learn2quote.html
An ideal world is left as an excercise to the reader.
--- Paul Graham, On Lisp 8.1