Need I to put overload or override words after the constructor declaration in derived class?

Question

I have a class hierarchy, this one:

type
TMatrix = class
    protected
      //...
    public
      constructor Create(Rows, Cols: Byte);
    //...
type
  TMinMa         


        

Answer 1

As far as I know, there are two separate issues here:

Making sure the child class' constructor calls the base class' constructor

You'll have to explicitly call the base class' constructor:

constructor TMinMatrix.Create(Rows, Cols: Byte);
begin
   inherited;
   //...
end;

Making sure the child class' constructor overrides the base class' constructor

You'll also have to make the child class' constructor override, and the base class' constructor virtual, to make sure the compiler sees the relation between the two. If you don't do that, the compiler will probably warn you that TMinMatrix's constructor is "hiding" TMatrix's constructor. So, the correct code would be:

type
TMatrix = class
    protected
      //...
    public
      constructor Create(Rows, Cols: Byte); virtual;    // <-- Added "virtual" here
      //...
type
  TMinMatrix = class(TMatrix)
    private
      //...
    public
      constructor Create(Rows, Cols: Byte); override;   // <-- Added "override" here
      constructor CreateCopy(var that: TMinMatrix);
      destructor Destroy; override;                     // <-- Also make the destructor "override"!
  end;

Note that you should also make your destructor override.

Introducing a constructor with different parameters

Note that you can only override a constructor with the same parameter list. If a child class needs a constructor with different parameters, and you want to prevent the base class' constructors from being called directly, you should write:

type
TMyMatrix = class(TMatrix)
//...
public
  constructor Create(Rows, Cols, InitialValue: Byte); reintroduce; virtual;
//...
end

implementation

constructor TMyMatrix.Create(Rows, Cols, InitialValue: Byte);
begin
  inherited Create(Rows, Cols);   // <-- Explicitly give parameters here
  //...
end;

I hope this makes things more clear... Good luck!