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Pointer to an abstract class


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#1 lucky6969b   Members   -  Reputation: 540

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Posted 21 May 2013 - 01:42 AM

#include "..\Objects\Objects.h"



class Idle;



class Activity

{

public:

    Objects *Actor;

    Goods *Target;

 

Objects and Goods are both abstract.

How can I declare an abstract object inside another class?

Thanks

Jack


Edited by lucky6969b, 21 May 2013 - 02:03 AM.


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#2 AgPF6   Members   -  Reputation: 152

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Posted 21 May 2013 - 02:03 AM

[code=auto:0]
class a
{
public:
class b
{
public:
virtual void something() = 0;
}
}
[\code]

Not sure why you would want to do this.

#3 NightCreature83   Crossbones+   -  Reputation: 2674

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Posted 21 May 2013 - 06:11 AM

You want to do that to make that class only accessible form that class usually, this is what a PIMPL implementation often uses, you should avoid pimpl it's a bad pattern. You shouldn't be hiding the data a class is operating on.

 

You can instantiate a pointer to an abstract class by newing an object instance of a derived class on it.

 

class base:
{
    virtual void abstractMethod() = 0;
}
 
class derived : public base
{
    virtual void abstractMethod()
    {
         cout << "fubar" << endl;
    }
}
void main()
{
    Base* derivedInstance = new derived();
    derivedInstance->abstractMethod();
    delete derived;
}

Worked on titles: CMR:DiRT2, DiRT 3, DiRT: Showdown, GRID 2, Mad Max

#4 Bregma   Crossbones+   -  Reputation: 4768

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Posted 21 May 2013 - 07:01 AM

You want to do that to make that class only accessible form that class usually, this is what a PIMPL implementation often uses, you should avoid pimpl it's a bad pattern. You shouldn't be hiding the data a class is operating on.

Say what?  This is the exact opposite of the advice I would give if I were to give good advice.
Stephen M. Webb
Professional Free Software Developer

#5 Bregma   Crossbones+   -  Reputation: 4768

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Posted 21 May 2013 - 07:06 AM

How can I declare an abstract object inside another class?

I'm not sure what you're trying to ask here.  Do you mean how do you instantiate objects of a concrete class derived from an abstract class?  Google for 'factory function'.


Stephen M. Webb
Professional Free Software Developer

#6 vleugel   Members   -  Reputation: 252

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Posted 21 May 2013 - 10:57 AM

You cannot produce an 'abstract object' at all, abstract classes are used for inheritance, as interface for a concrete class derived from it.

If Object and Goods are in fact not abstract, just use 'Target = new Goods()' and 'Actor = new Objects()' into the constructor or something if thats what you want (although I would probably just use static objects by deleting the asterisk if thats your plan).



#7 brx   Members   -  Reputation: 683

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Posted 21 May 2013 - 03:30 PM

You want to do that to make that class only accessible form that class usually, this is what a PIMPL implementation often uses, you should avoid pimpl it's a bad pattern. You shouldn't be hiding the data a class is operating on.


Not trying to derail the topic (sorry), but I'd like to hear more about your reasoning for this statement (the parts I've emphasized).

IMO there's nothing inherently wrong about the PIMPL pattern (except that it produces a lot of boiler plate code - but at the same time it enables nice things like implicit sharing and reducing compile times).
Besides, declaring the data class inside the actual class would not really be the PIMPL pattern, as the PIMPL would still be "public" (not in the language kind of way, but in the "I see what you did there" kind of way) in C++ as it could be seen in the header (you'd usually have a private header that's only included in the implementation not in the declaration; also, only when used like that, the PIMPL pattern can reduce compile time, which is one of its main merrits).
[EDIT - _very_ exaggerated, to be honest]: Actually, isn't PIMP the only true object orientated pattern? Afterall, one of the main points is encapsulation. What's more encapsulated than only showing the actual interface to the user? The user should not care "how" and "what data", but only "what can I do with it?".

Back to the topic: to me the question is lacking some information. What are you trying to accomplish? To me it seems like you are actually asking how to instanciate objects of the "Objects" and "Goods" classes (btw, get rid of the plural form)!? If that is the case, you will need actual implementation of the abstract classes. This is a long shot, though, since some information is missing here.

Edited by brx, 21 May 2013 - 03:40 PM.





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