Base Class pointers to Derived Classes calling functions.

[bgold]

Hey. I have a base class (SPRITE), and using this base class I have
derived a large number of derived classes (PERSON, BULLET, MISSILE,
etc.). Now, at a certain point in my program, I have a pair of
pointers, where each is a pointer to the base class (each is a SPRITE
*). I know that each of these pointers actually points to one of the
derived classes, even though the type of the pointer is SPRITE *, but
I don't know which derived class it points to. I would like to write a
set of functions that do something different depending on what
combination of derived classes the two pointers point to (i.e. I want
one function for a PERSON * and BULLET *, a second function for a
PERSON * and MISSILE *, a third for BULLET * and MISSILE *, etc.).

How can I write these functions, and how do I call them, so that the
correct function is called given the two pointers?

I hope this is clear...

 

[sonison.james]

Hey. I have a base class (SPRITE), and using this base class I have
derived a large number of derived classes (PERSON, BULLET, MISSILE,
etc.). Now, at a certain point in my program, I have a pair of
pointers, where each is a pointer to the base class (each is a SPRITE
*). I know that each of these pointers actually points to one of the
derived classes, even though the type of the pointer is SPRITE *, but
I don't know which derived class it points to. I would like to write a
set of functions that do something different depending on what
combination of derived classes the two pointers point to (i.e. I want
one function for a PERSON * and BULLET *, a second function for a
PERSON * and MISSILE *, a third for BULLET * and MISSILE *, etc.).

How can I write these functions, and how do I call them, so that the
correct function is called given the two pointers?

I hope this is clear...

Not sure if what you want is an overloaded function to automatically
figure out the correct overload based on the pointer; if that's the
case then I don't think it would be possible with pointers.

You could however, use dynamic_cast(or even typeid) to figure out what
the pointer is pointing to and then make the appropriate set of calls
in the code. Though this design would not scale well as you add more
classes.

Thanks and regards
Sonison James

 

[Daniel T.]

Hey. I have a base class (SPRITE), and using this base class I have
derived a large number of derived classes (PERSON, BULLET, MISSILE,
etc.). Now, at a certain point in my program, I have a pair of
pointers, where each is a pointer to the base class (each is a SPRITE
*). I know that each of these pointers actually points to one of the
derived classes, even though the type of the pointer is SPRITE *, but
I don't know which derived class it points to. I would like to write a
set of functions that do something different depending on what
combination of derived classes the two pointers point to (i.e. I want
one function for a PERSON * and BULLET *, a second function for a
PERSON * and MISSILE *, a third for BULLET * and MISSILE *, etc.).

How can I write these functions, and how do I call them, so that the
correct function is called given the two pointers?

I hope this is clear...

Use the Visitor pattern. Better yet, redesign your program so you don't
have such a situation.

Wikipedia has a nice article on the Visitor Pattern, including sample
code (in Java.) Check it out.

 

[Rahul]

Not sure if what you want is an overloaded function to automatically
figure out the correct overload based on the pointer; if that's the
case then I don't think it would be possible with pointers.

You could however, use dynamic_cast(or even typeid) to figure out what
the pointer is pointing to and then make the appropriate set of calls
in the code. Though this design would not scale well as you add more
classes.

Thanks and regards
Sonison James

dynamic_cast would work only with classes having virtual functions,
typeid operator would
be more appropriate...

 

[Rahul]

Hey. I have a base class (SPRITE), and using this base class I have
derived a large number of derived classes (PERSON, BULLET, MISSILE,
etc.). Now, at a certain point in my program, I have a pair of
pointers, where each is a pointer to the base class (each is a SPRITE
*). I know that each of these pointers actually points to one of the
derived classes, even though the type of the pointer is SPRITE *, but
I don't know which derived class it points to. I would like to write a
set of functions that do something different depending on what
combination of derived classes the two pointers point to (i.e. I want
one function for a PERSON * and BULLET *, a second function for a
PERSON * and MISSILE *, a third for BULLET * and MISSILE *, etc.).

How can I write these functions, and how do I call them, so that the
correct function is called given the two pointers?

I hope this is clear...

Is this a global function or are you planning to have it as a member
function, is so in which class?

 

[bgold]

Is this a global function or are you planning to have it as a member
function, is so in which class?

This is a set of global functions, each of which will take a different
pair of pointers.

I'd rather not use typeid. Isn't there some way of doing overloaded
functions so that I can just have one call that automatically calls
the correct function (for example: OverloadedFunction(PERSON *p,
BULLET *bt); and OverloadedFunction(PERSON *p, MISSILE *ms); etc.)?

 

[Daniel T.]

[paraphrase] I need to implement double dispatch but I don't
know how.

Is this a global function or are you planning to have it as a
member function, is so in which class?

This is a set of global functions, each of which will take a
different pair of pointers.

I'd rather not use typeid. Isn't there some way of doing overloaded
functions so that I can just have one call that automatically calls
the correct function (for example: OverloadedFunction(PERSON *p,
BULLET *bt); and OverloadedFunction(PERSON *p, MISSILE *ms); etc.)?

No. The Visitor pattern or some variant of it (Acyclic Visitor comes to
mind) is your only option.

http://www.objectmentor.com/resources/articles/acv.pdf

 

[jkherciueh]

This is a set of global functions, each of which will take a different
pair of pointers.

I'd rather not use typeid. Isn't there some way of doing overloaded
functions so that I can just have one call that automatically calls
the correct function (for example: OverloadedFunction(PERSON *p,
BULLET *bt); and OverloadedFunction(PERSON *p, MISSILE *ms); etc.)?

Not really. The problem is known as "double dispatch" and there are several
ways to deal with it, but all of them require a certain amount of
scaffolding and support from the classes involved. If C++ had templated
virtual member functions, one could (using some trickery) encode tables to
resolve the calls within the vtables; but since that support is missing
from the language, the usual solutions either involve handcoding the double
dispatch through member functions or setting up a static table manually. I
am sure that google will provide plenty of information about double
dispatch so that you can make an informed decision on how you want to go
about it.


Best

Kai-Uwe Bux

 

[bgold]

Looks like I'm going to have to figure out how to implement this
Visitor pattern, if I can't entirely restructure my program. Thanks
everyone.

 

[Ron Natalie]

dynamic_cast would work only with classes having virtual functions,
typeid operator would
be more appropriate...

typeid has the same restrictions. If the class isn't polymorphic
(virtual) functions, the runtime typing info ain't there.

 

[Rahul]

typeid has the same restrictions. If the class isn't polymorphic
(virtual) functions, the runtime typing info ain't there.

I meant the compilation error, one gets with dynamic_cast for the
following cases,

class A
{
};

class B : public A
{
};

int main()
{
B *ptr1;
A *ptr2 = new B();
ptr1 = dynamic_cast<B*>(ptr2); // gives a compilation
error
if(ptr1 == NULL)
printf("fail\n");
else
printf("success\n");
return(0);
}

but typeid doesn't give any such error,

A *ptr = new B();
cout<<typeid(ptr).name()<<endl;
cout<<typeid(*ptr).name()<<endl; // Yes this would give A
without any virtual functions in A, but atleast works...

 

[Ron Natalie]

I meant the compilation error, one gets with dynamic_cast for the
following cases,

ptr1 = dynamic_cast<B*>(ptr2); // gives a compilation
error

If your compiler gives a compilation error there it is seriously busted.
Dynamic cast has a whole slew of cases that apply EVEN if the classes
aren't polymorphic.

A warning perhaps, but the program is well formed.

 

[James Kanze]

If your compiler gives a compilation error there it is
seriously busted. Dynamic cast has a whole slew of cases that
apply EVEN if the classes aren't polymorphic.

True, but pointer to base to pointer to derived isn't one of
them. The above code should result in a compiler error.
(Remember that ptr2 has type A*, and that A is a base class of
B.)

The reverse, of course, should work.