Backwards inheritance? - GameCreators Forum

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Hawkblood

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Posted: 15th Oct 2010 19:25

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This might be beyond most of you (it's beyond me so far). If you have two classes:

+ Code Snippet

class A{
public:
int stupid;
void ShowStupid(void);
};
class B{
public:
int x,y;
A MeStupid;
};

Class "B" inherits class "A"'s info and can be accessed from within class "B":

+ Code Snippet

B Hello;
Hello.MeStupid.stupid=5;

But, I can't do this:

+ Code Snippet

B Hello;
Hello.MeStupid.ShowStupid();

// and within the "A" class "ShowStupid()" is like this:
void A::ShowStupid(void){
dbText(x,y,dbStr(stupid));
}

Because "x,y" is part of "B" not "A".

So here is my question: Is there a way to get info from "B" to be used within "A"? Remember, if I try this within "A"

+ Code Snippet

void ShowStupid(B *pB);

It will fail to compile because class B has yet to be declared.

Any thoughts?

The fastest code is the code never written.

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Greg_C

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Posted: 15th Oct 2010 20:46 Edited at: 15th Oct 2010 20:47

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The whole point of classes is inheritance and polymorphism. If you find yourself wanting to use a function that is in class B in class A then your class hierarchy is all screwed up. The point is to be able to change a function in your base class and all of the classes that inherit from it get those changes as well. So if you want to use a function in A that is in B just make the function in A and then you can use it in A and B.

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Hassan

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Posted: 15th Oct 2010 20:56

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Quote: "It will fail to compile because class B has yet to be declared."

forward declaration would help on this

But as Greg_C said, doing such thing means that the structure is screwed up, you need to rethink it

also, you probably know this, but anyway, inheritance is not done like:

+ Code Snippet

class A
{
protected:
int x;
};
class B
{
A classA;
};

now B doesn't inherit anything, instead it has an instance of an A object, it should be like:

+ Code Snippet

class A
{
protected:
int x;
};
class B : public A
{
int y;
};

and accessing is not done like:

+ Code Snippet

B b;
b.classA.x;

instead, directly

+ Code Snippet

B b;
b.x;

as if x was defined inside B, not A

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Mireben

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Posted: 15th Oct 2010 21:26

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The simplest solution is to parameterize your ShowStupid function and call it with the coordinates:

+ Code Snippet

B Hello;
Hello.MeStupid.ShowStupid(Hello.x, Hello.y);

void A::ShowStupid(int x, int y){
dbText(x,y,dbStr(stupid));
}

But if the classes are so interrelated that A needs data from B and B needs data from A, why not combine them into the same class?

Your design is not inheritance as others already pointed out, it is containment: B contains A (also a valid relationship). However it doesn't matter which relationship it is, a contained class will not access the container's data and a base class will not access data of a derived class.

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Hawkblood

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Posted: 16th Oct 2010 17:03

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Well, I did a workaround. I put the "showstupid()" in B instead of A.
I was hoping there would be a way to make it happen. I tried to make a class declaration before 'A' the same way you declare functions "void something(void);" prior to making any calls to "something();". It compiled, but failed in execution.

There are C++ keywords like "this" which seems to be kinda redundant because it refers to the class that it's in. I was hoping there was something like "this" for the parent class.

Ah, whishful thinking.

The fastest code is the code never written.

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Matty H

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Posted: 16th Oct 2010 17:25

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Quote: "I was hoping there was something like "this" for the parent class."

There is:

+ Code Snippet

void someChildFunction()
{
    parentClass::someParentFunction();
}

Also, you do realise that you are not using inheritance in your examples right? So the above will not work unless you use inheritance:

+ Code Snippet

class A{
}

class B : public A{
}

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Hawkblood

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Posted: 16th Oct 2010 20:12

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@matty halewood
When I get more time, I'll have to study it. There's a lot of things about C++ that I still don't know.

The fastest code is the code never written.

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Hawkblood

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Posted: 18th Oct 2010 18:39

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I think I've found something that would do what I was wanting:
http://www.cplusplus.com/doc/tutorial/inheritance/

The keyword is "friend".

The fastest code is the code never written.

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