Den 23-02-2014 15:26, Mark Morgan Lloyd skrev:
I think I've sorted this but I'd appreciate being kicked if it looks
like I'm doing something badly wrong.
What determines the constructor that's called when an object's created?
I've got a couple of classes defined like this:
TB5500BaseUnit=
class(TObject)
constructor Create(enum: integer; mnem: String7; unitIndex:
integer; unitDesignate: integer; cc: TObject);
..
TB5500SPOUnit=
class (TB5500BaseUnit)
constructor Create(enum: integer; mnem: String7; unitIndex:
integer; unitDesignate: integer; cc: TObject);
..
The actual objects being created are in response to configuration
tables and files:
var
unitClass: class of TB5500BaseUnit; (* Ancestor of all I/O devices *)
u: TB5500BaseUnit;
name: string; (* For debugging *)
..
unitClass := LookupUnitType( { from device name } );
name := unitClass.ClassName; (* For debugging *)
name := ''; (* Make sure it's wiped *)
u := unitClass.Create(Ord(mnem), specs.unitName,
specs.unitIndex, specs.designate, self);
name := u.ClassName; (* For debugging *)
thisUnit[specs.unitIndex] := u;
If I was trying to create a TB5500SPOUnit I was finding that unitclass
was set correctly, and that u was getting a valid type with (later)
public methods declared as overrides being available. The odd thing
was that TB5500SPOUnit.Create() was not being called, but instead
TB5500BaseUnit.Create().
I find that if I explicitly decorate TB5500BaseUnit.Create() as
virtual and TB5500SPOUnit.Create() as override then
TB5500SPOUnit.Create() is called correctly. I didn't think this was
necessary, but on reflection I assume that it's because u is declared
as a TB5500BaseUnit: the variable's compile-time rather than run-time
class is being used for the constructor unless explicitly overridden.
Is there an obvious way of tiying this up that I'm overlooking?
You have forgotten to declare the constructor virtual.
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