Databases
StackOverflow2013
Postsdbo
POAvoiding casting when passing objects through library code (In Delphi)
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POAvoiding casting when passing objects through library code (In Delphi)
    primarykey
    data
    text
    <p>When designing libraries, I often end up resorting to the following pattern, which I don't like as it results in lots of type-casting.<br></p> <p>The basic pattern is:<br> The code using the library hands an object to the library, the library then hands the object back to the calling code. The calling code is forced to cast the object, as the library hands back a generic type. (Stripped-down code example below)</p> <p>The library defines the following objects and function:</p> <pre><code>TThing = Class End; TThingProcessor = Class Public Function CreateThing : TThing; Virtual; Abstract; Procedure ProcessThing (Thing : TThing); Virtual; Abstract; End; Procedure DoEverything (Processor : TThingProcessor); </code></pre> <p>The calling code then uses the library by overriding the objects and calling DoEverything, as follows -</p> <pre><code>TMyThing = Class(TThing) Public X : Integer; End; TMyThingProcessor = Class(TThingProcessor) Public XSum : Integer; Function CreateThing : TThing; Override; Procedure ProcessThing (Thing : TThing); Override; End; Function TMyThingProcessor.CreateThing : TThing; Begin Result := TMyThing.Create; End; Procedure TMyThingProcessor.ProcessThing (Thing : TThing); Begin XSum := XSum + (Thing As TMyThing).X; //Here is the problem, the caller is forced to cast to do anything End; </code></pre> <p>The processor class is also a TThing factory. The library guarantees that it will only pass TThings to the corresponding TThingProcessor that created them, so it works, but isn't type-safe. While the code above is a bit stupid in that it doesn't really do anything, it shows why ProcessThing can't simply be shifted to TThing and be polymorphic - the XSum variable needs to be updated.</p> <p>How can I restructure the code so the cast is unnecessary? I need to keep the library code separate but be able to accept any type.</p> <p>Edit: Changed the hard-cast to an as-cast due to suggestion so it will at least throw exception instead of crash in the case of mismatched types</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDavid
    1. USDavid
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POAvoiding casting when passing objects through library code (In Delphi)
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POAvoiding casting when passing objects through library code (In Delphi)
      1. USLieven Keersmaekers
      1. VTFavorite
    3. VO
      singulars
      1. POAvoiding casting when passing objects through library code (In Delphi)
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI think that http://stackoverflow.com/questions/681522/casting-between-parent-and-child-classes-in-delphi" might have some bearing on what you're trying to do
      singulars
      1. POAvoiding casting when passing objects through library code (In Delphi)
      1. USRobS
    2. COThanks for the link, it was useful reading. I don't think it applies directly but thinking about restructuring in a similar way is throwing up ideas for me, though nothing I've been able to pin down yet
      singulars
      1. POAvoiding casting when passing objects through library code (In Delphi)
      1. USDavid
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload