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PODELPHI: Generics and polymorphism
 

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  1. PODELPHI: Generics and polymorphism
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    <p>This has been asked several different ways already - but I haven't found my answer yet.</p> <p>Can someone clarify a few things for me please. Using : Delphi XE2</p> <p>I have quite a big BaseObject that I use for almost everything. Along with it I have a Generic list - BaseList.</p> <p>Delarations go like this :</p> <pre><code>TBaseObject = class ... a lot of properties and methods ... end; TBaseList&lt;T: TBaseObject&gt; = class(TObjectList&lt;T&gt;) ... some properties and methods ... end; </code></pre> <p>I have recently tried to change the TBaseList declaration from a very old TStringList using Objects[] property... to this never more versatile Generics list TObjectList.</p> <p>But I run into some problems. The BaseUnit is one file ... and every time I descend my BaseObject I also make a specialized list to follow it.</p> <p>So I would go and do something like :</p> <pre><code>TCustomer = class(TBaseObject) ... customer related stuff ... end; TCustomerList&lt;T: TCustomer&gt; = class(TBaseList&lt;T&gt;) ... customer list specific stuff ... end; </code></pre> <p>But now I would like an object to contain a list - that can hold any object. And I thought I could do it like this</p> <pre><code>TControlObject = class(TBaseobject) FGenList: TBaseList&lt;TBaseObject&gt;; end; </code></pre> <p>Since BaseList and BaseObject is top of my hierarchy I assumed that such a List would be able to hold any list I could think of.</p> <p>But I have a feeling that it is here I fail ... a <code>TBaseList&lt;TBaseobject&gt;</code> is somehow not comparable to <code>TCustomerList&lt;TCustomer&gt;</code> ... Even if <code>TCustomerList</code> and <code>TCustomer</code> is descended from my base.</p> <p>I was hoping to be able to use generics in the baselist for instaciating new objects. ie. using <code>T.Create</code> in a populate method. </p> <p>Here is example of complete hierarchy:</p> <pre><code>Base Unit; TBaseObject = class end; TBaseList&lt;T:TBaseObject&gt; = class(TObjectList&lt;T&gt;) end; CustomCustomer Unit; TCustomCustomer = class(TBaseObject) end; TCustomCustomerList&lt;T:TCustomCustomer&gt; = class(TBaseList&lt;T&gt;) end; Customer Unit; TCustomer = class(TCustomCustomer) end; TCustomerList&lt;T:TCustomer&gt; = class(TCustomCustomerList&lt;T&gt;) end; CustomPerson Unit; TCustomPerson = class(TBaseObject) end; TCustomPersonList&lt;T:TCustomPerson&gt; = class(TBaseList&lt;T&gt;) end; Person Unit; TPerson = class(TCustomPerson) end; TPersonList&lt;T:TPerson&gt; = class(TCustomPersonList&lt;T&gt;) end; </code></pre> <p>Given the above hierarchy - why can't I :</p> <pre><code>var aList : TBaseList&lt;TBaseObject&gt;; // used as a list parameter for methods aPersonList : TPersonList&lt;TPerson&gt;; aCustomerList : TCustomerList&lt;TCustomer&gt;; begin aPersonList := TPersonList&lt;TPerson&gt;.Create; aCustomerList := TCustomerList&lt;TCustomer&gt;.Create; aList := aCustomerList; &lt;-- this FAILS !! types not equal .. end; </code></pre> <p>Calling a procedure that handles the base class for all lists fails the same way ...</p> <pre><code>Procedure LogStuff(SomeList : TBaseList&lt;TBaseObject&gt;) begin writeln(Format( 'No. Elements in list : %d',[SomeList.Count])); end; </code></pre> <p>Can someone punch me and tell me what I'm doing wrong here?</p>
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    1. COCan you give some code where you see the failing of this aproach? I'm not sure I've got it.
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      1. PODELPHI: Generics and polymorphism
      1. USUwe Raabe
    2. COAre you trying to assign a TCustomerList to FGenList, or are you trying to store TCustomer's in FGenList? (Not that I would then have an answer, but it may clarify what you are trying to achieve). Also, why are you declaring different list types? The point of generics is that you no longer need them for type safety. TList<TCustomer>.Create is type incompatible with TList<TProduct>.Create.
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      1. USMarjan Venema
    3. CO@Uwe and Marjan : I've edited my question ... grown quite big - I hope it still has context. But perhaps its more understandable now?
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