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PODelphi: When does reintroduce hide ancestors and when does it show them?
 

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  1. PODelphi: When does reintroduce hide ancestors and when does it show them?
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    <p><strike>Today</strike> Recently on Stackoverflow i learned that:</p> <ul> <li><a href="https://stackoverflow.com/questions/3874330/delphi-how-to-hide-ancestor-constructors">reintroduce is used to <strong><em>hide</em></strong> ancestor constructors</a></li> <li><a href="https://stackoverflow.com/questions/3876484/delphi-how-to-add-a-different-constructor-to-a-descendant">reintroduce is used to <strong><em>show</em></strong> ancestor constructors</a></li> </ul> <p>i've been trying to make sense of it all, so here is a another, very specific question, <a href="https://stackoverflow.com/questions/3876040/delphi-understanding-constructors">supporting my main question dealing with constructors</a>.</p> <hr> <p><strong>Update:</strong> replaced the entire question:</p> <pre><code>TComputer = class(TObject) public constructor Create(Teapot: string=''); end; TCellPhone = class(TComputer) public constructor Create(Cup: Integer); overload; virtual; constructor Create(Cup: Integer; Teapot: string); overload; virtual; end; </code></pre> <p>When constructing TCellPhone, 3 constructors are avaible: </p> <ul> <li>Cup: Integer</li> <li>Cup: Integer; Teapot: string</li> <li>[Teapot: String = '']</li> </ul> <p>Question: Why is <code>constructor(Teapot: string='')</code> not being hidden?</p> <hr> <p>Now i added a 3rd descendant:</p> <pre><code>TComputer = class(TObject) public constructor Create(Teapot: string=''); end; TCellPhone = class(TComputer) public constructor Create(Cup: Integer); overload; virtual; constructor Create(Cup: Integer; Teapot: string); overload; virtual; end; TiPhone = class(TCellPhone) public constructor Create(Cup: Integer); override; end; </code></pre> <p>When constructing <code>TiPhone</code> <strong><em>four</em></strong> constructors are available:</p> <ul> <li>Cup: Integer</li> <li>Cup: Integer</li> <li>Cup: Integer; Teapot: string</li> <li>[Teapot: string = '']</li> </ul> <p>Why are there <strong><em>four</em></strong> constructors? i overrode one of the existing three. <strong>Edit:</strong> This may be a bug in code-insight, it shows me four - yet how could i possibly call then when two are the same.</p> <hr> <p>Using the original code again:</p> <pre><code>TComputer = class(TObject) public constructor Create(Teapot: string=''); end; TCellPhone = class(TComputer) public constructor Create(Cup: Integer); overload; virtual; constructor Create(Cup: Integer; Teapot: string); overload; virtual; end; </code></pre> <p>it's already known that <code>TCellPhone</code> has three constructors: </p> <ul> <li>Cup: Integer</li> <li>Cup: Integer; Teapot: string</li> <li>[Teapot: String = '']</li> </ul> <p>How do i alter the declaration of <code>TCellPhone</code> to hide the ancestor constructor? e.g. so that:</p> <pre><code>TNokia = class(TCellPhone) end; </code></pre> <p>will only have two constructors:</p> <ul> <li>Cup: Integer</li> <li>Cup: Integer; Teapot: string</li> </ul> <hr> <p>Now for the case where <code>reintroduce</code> is used to hide a non-virtual ancestor. In the previous case <code>TiPhone</code> has four constructors (ideally there would be only two - with <code>TComputer</code> somehow hiding its ancestor). But even if i can't fix <code>TComputer</code>, i can change <code>TiPhone</code> to only have the one:</p> <pre><code>TComputer = class(TObject) public constructor Create(Teapot: string=''); end; TCellPhone = class(TComputer) public constructor Create(Cup: Integer); overload; virtual; constructor Create(Cup: Integer; Teapot: string); overload; virtual; end; TiPhone = class(TCellPhone) public constructor Create(Cup: Integer); reintroduce; end; </code></pre> <p>Now <code>TiPhone</code> has only one constructor:</p> <ul> <li>Cup: Integer</li> </ul> <p>Reintroduce is normally only used to suppress the warning about hiding <strong>virtual</strong> ancestors. In this case:</p> <pre><code>Create(Teapot: string = '') </code></pre> <p>isn't virtual - yet i can still use reintroduce to hide it.</p> <hr> <p>But <strong>now</strong>, if i add another overloaded to <code>TiPhone</code>:</p> <pre><code>TiPhone = class(TCellPhone) public constructor Create(Cup: Integer); reintroduce; overload; constructor Create(Handle: String); overload; end; </code></pre> <p>Then suddenly the (previously hidden) ancestors come back:</p> <ul> <li>TiPhone.Create(7);</li> <li>TiPhone.Create('pink');</li> <li>TiPhone.Create(7, 'pink');</li> <li>TiPhone.Create();</li> </ul> <p>As you can see, i'm struggling to understand the logic of</p> <ul> <li>when something is hidden</li> <li>how to hide something</li> <li>when something is shown</li> <li>how to show something</li> </ul>
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    1. USIan Boyd
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      1. PODelphi: When does reintroduce hide ancestors and when does it show them?
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      1. USJeroen Wiert Pluimers
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    1. CO@Ian: You really do ask a lot of questions today! Nice!
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      1. PODelphi: When does reintroduce hide ancestors and when does it show them?
      1. USAndreas Rejbrand
    2. COAre you really saying you can compile run `obj := TCellPhone.Create('foo')`? I find that surprising.
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      1. PODelphi: When does reintroduce hide ancestors and when does it show them?
      1. USRob Kennedy
    3. COI have certainly run a full example using these definition. Luckily, we already have the answer below.
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      1. PODelphi: When does reintroduce hide ancestors and when does it show them?
      1. USMuhammad Alkarouri
 

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