CODEWITHSUNDEEP
delphigenericsdelphi-2010
Phillip Roux
Phillip Roux

Reputation: 173

Structure Delphi Generics Class

I'm new to generics and need some help to structure a class and implement methods.

I'm trying to use generics to serialize any TObject-JSON. Moreover, I want to be able to reuse the code.

These are my questions:

  1. How do I create a generic constructor? I want to be able to use Self or Default(T), but it returns just nil.

  2. V := Marshal.Marshal(ReturnObject) - This method requires a TObject, but I do not know how to reference the current object that was passed in.

  3. How can I use this inside a method? Look at the code snipped below, marked with "Question 3".

This is my code:

TFileOperationResult = class(TObject)
private
  FSuccess: Boolean;
  //Error: PException;
  FLastError: Integer;
  function GetFailure: Boolean;
  property Failure: Boolean read GetFailure;
public
  property Success: Boolean read FSuccess write FSuccess;
  property LastError: Integer read FLastError write FLastError;
end;

TResponseObject<T: class> = class(TObject)
private
  FReturnObject: T;
  function GetReturnObject: T;
  function BaseStringsConverter(Data: TObject): TListOfStrings;
public
  constructor Create; overload;
  property ReturnObject: T read GetReturnObject;
  procedure Serialize;
end;

constructor TResponseObject<T>.Create;
begin
 // Question 1 - What should go in here?
end;

function TResponseObject<T>.GetReturnObject: T;
begin
  Result := Default(T);// Is this correct?
end;

procedure TResponseObject<T>.Serialize;
var
  Marshal: TJSONMarshal;
  V: TJSONValue;
begin
  Marshal := TJSONMarshal.Create(TJSONConverter.Create);
  Marshal.RegisterConverter(TResponseObject<T>, BaseStringsConverter);
  V := Marshal.Marshal(ReturnObject); // Question 2 - How Can I refer to 'Self'?
  OutPut := V.ToString;
  Marshal.Free;
end;

Calling code:

procedure TForm1.Test;
var
  FileOperationResult: TResponseObject<TFileOperationResult>;
begin
  FileOperationResult := TResponseObject<TFileOperationResult>.Create;
  FileOperationResult.Serialize;
end;

Question 3:

procedure TForm1.MoveCopyFile<THowNowResponse>(ASource, DDestination: String);
var
  FileOperationResult: TFileOperationResult;
begin
  FileOperationResult := TFileOperationResult.Create;
  // What to do?
end;

Any other comments are much appreciated.

Upvotes: 2

Views: 1154

Answers (1)

Mason Wheeler
Mason Wheeler

Reputation: 84550

It's hard to tell exactly what you're trying to do here, but I can make a guess. For TResponseObject, you want an object that can contain another object and operate on it. In that case, you probably want to pass it in to the constructor, like so:

constructor TResponseObject<T>.Create(value: T);
begin
  FReturnObject := value;
end;

Likewise, if you make a GetReturnObject method, it should probably return the value of the FReturnObject field. (Or you could make the read accessor of the property just reference FReturnObject directly.)

function TResponseObject<T>.GetReturnObject: T;
begin
  Result := FReturnObject;
end;

It's really hard to answer #3 since I don't know what you're trying to do with this, but hopefully my answers to the first two will help you get back on track. Just remember that generics don't have to be confusing; they're basically just type substitution. Anywhere you'd use a normal type in a non-generic routine, you can replace it with a <T> to create a generic routine, and then substitute any type that fits the constraints for that particular T.

Upvotes: 1

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