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    <p>I've written about this before, using an example very similar to <a href="https://stackoverflow.com/questions/2051673/how-to-safely-bypass-delphi-error-types-of-formal-and-actual-parameters-must-be/2051769#2051769">Lasse's</a>:</p> <ul> <li><a href="http://www.cs.wisc.edu/~rkennedy/var-identical" rel="nofollow noreferrer">Delphi Q&amp;A: Why must the types of actual and formal var parameters be identical?</a></li> </ul> <p>Unless you're writing an assignment statement to <em>change the value</em> of the input parameter itself, and not just one of its properties, you shouldn't pass a parameter by reference in the first place.</p> <p>If you <em>are</em> writing an assignment statement to change the parameter's value, then the compiler message really is true, and you should heed it.</p> <p>One reason for needing to by-pass the error is when you're writing a function like <a href="http://docwiki.embarcadero.com/VCL/en/Forms.TApplication.CreateForm" rel="nofollow noreferrer"><code>TApplication.CreateForm</code></a>. Its job is to change the input parameter's value, and the type of the new value varies and cannot be determined at compile time. If you're writing such a function, then your only option with Delphi is to use an <a href="http://www.cs.wisc.edu/~rkennedy/untyped" rel="nofollow noreferrer"><em>untyped var</em></a> parameter, and then there is extra burden on both the caller and the receiver to make sure everything goes right. The caller needs to make sure it passes a variable that is capable of holding values of whatever type the function will put in it, and the function needs to make sure it stores a value of a type compatible with what the caller requested.</p> <p>In the case of <code>CreateForm</code>, the caller passes in a class-reference literal and a variable of that class type. The function instantiates the class and stores the reference in the variable.</p> <p>I don't think very highly of either <code>CreateForm</code> or <code>FreeAndNil</code>, largely because of the way their untyped parameters sacrifice type safety in return for comparatively little extra convenience. You haven't shown the implementation of your <code>TotalDestroy</code> function, but I suspect its var parameter will ultimately provide the same low utility as in those other two functions. See my articles on both:</p> <ul> <li><a href="http://www.cs.wisc.edu/~rkennedy/freeandnil" rel="nofollow noreferrer">When should I use <code>FreeAndNil</code>?</a></li> <li><a href="http://www.cs.wisc.edu/~rkennedy/createform" rel="nofollow noreferrer">Why shouldn't I call <code>Application.CreateForm</code>?</a></li> </ul>
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    1. USRob Kennedy
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    1. COAs you cleverly guessed, my TotalDestroy does some special clean up of TMyObject and finally does a FreeAndNil.
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    2. COIf there's special cleanup required for a `TMyObject`, consider putting that in the destructor. Don't force consumers of your class to learn a new way of destroying it when plain old `Free` works fine for everything else.
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    3. COThanks Rob. I take your point, I don't need my TotalDestroy. I will go just with my Destroy destructor I already have. And let the user decide if they want it thru FreeAndNil or not.
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