Re: Dealing with a Diamond of Death
Pete Becker wrote:
On 2008-10-27 18:08:43 -0400, Rocketmagnet <hugo.elias@virgin.net> said:
Hi all,
I have been kind of forced (honestly) into writing a class structure
which contains a Diamond of Death, and I'm not entirely sure what to
do about it.
I don't understand. Having a common virtual base class is sometimes an
appropriate design. Why do you call it a "Diamond of Death"?
This is a simplified version of my class structure:
Simplified to the point of incorrectness, since the code in main (that's
what _tmain means, right?) refers to ENTITY but the nearest thing here
is entity.
class entity
{
public:
entity() {}
int a;
};
class item : public entity
{
public:
item() {}
int b;
};
class loop : public virtual item
{
public:
loop() {}
int c;
};
class path : public virtual item
{
public:
path() {}
int d;
};
class test : public path, public loop
{
public:
test() {}
int e;
};
int _tmain(int argc, _TCHAR* argv[])
{
ENTITY *entity_test = new test();
ENTITY *entity_loop = new loop();
loop* l = (loop*)entity_loop; // cannot convert a 'ENTITY*' to a
'loop*'
// conversion from a virtual base
class is implied
You need to use dynamic_cast here, and in order to do that, ENTITY or
entity, whichever is the real class, needs to have at least one virtual
function.
There are different ways to do it, C++ has a way, without resorting to
lower level languages with the platform runtime.
It is in the C++ FAQ how to deal with the dreaded diamond, I suggest you
read it.
http://www.parashift.com/c++-faq-lite/
http://www.google.com/search?q=diamond&sitesearch=www.parashift.com
In the C++ there is the virtual keyword in the application of the
reinheritance and so on. It defaults the class body's inherited method
from the common ancestor class, they can be casted among the
intermediate types.
Thanks,
Ross F.