[Qt-interest] QHash<QString,class>?

[Israel Brewster]

On Jan 12, 2009, at 9:21 AM, Bachrach, Robert L wrote:

> True, this is more of a C++ question than Qt.
>
> You are essentially referring to a variant of a factory method.  I  
> have seen this type of thing implemented in 2 ways:
>
> 1 - Create a pure virtual function in your base class, NewWindow,  
> that returns a pointer to a base class object.  Each derived class  
> overrides this function, returning a pointer to a new object of its  
> own type.  Then, your hash would contain an instance of each derived  
> class.  It simply calls the NewWindow function on the element in the  
> hash.  Of course, each base class would have to be "registered" by  
> having an instance placed in the hash.  Registration could occur in  
> the constructor, since you always create one.  Don't forget to  
> delete the objects when you clear the hash.
>
> 2 - Probably easier to manage is to create a static member function  
> of each derived class "NewWindow".  This static member function acts  
> as the virtual function above.  The hash simply stores a pointer to  
> each function.  This requires less memory and less memory cleanup,  
> but lacks the compiler error if you forget to declare the function  
> in a derived class.  It still requires registration.
>
> Although manual registration is fine, automatic registration can be  
> performed by instantiating a light weight object as a static class  
> variable.  The object's constructor could perform the registration.
>
> Please consider using C++ newsgroups for similar questions in the  
> future.
>
> Rob

Thanks for the detailed and helpful response. I actually did consider  
using a C++ newsgroup, but was thinking/hoping this was just a QHash  
question, i.e. getting the proper data type for the value (thus the  
subject). As that is apparently not the case, you are right: this does  
belong more on a C++ newsgroup. My apologies.
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Israel Brewster
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Frontier Flying Service Inc.
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