How can Delphi records be initialized automatically?

问题

To initialize Delphi records I've always added a method (class or object) that would initialize to known good defaults. Delphi also allows for defining record "constructors" with parameters, but you cannot define your own parameter-less "constructor".

TSomeRecord = record
 Value1: double;
 Value2: double;

 procedure Init;
end;

procedure TSomeRecord.Init;
begin
  Value1 := MaxDouble;
  Value2 := Pi;
end;

Given the record above there is no warning that a record has not been initialized. Developers may neglect to call Init on the record. Is there a way to automatically initialize records to my default, potentially more than just a simple FillChar;

For instance

var 
  LSomeRecord: TSomeRecord
begin
  // someone forgot to call LSomeRecord.Init here
  FunctionThatTakesDefaultSomeRecord(LSomeRecord);
end;

How can a record be initialized to my defaults automatically?

[Note] I don't want to modify the problem after it has been answered. Any readers are directed to read the comments on best practices on using class methods for initialization instead of a mutating object method.

回答1:

You can use a hidden string field (which is automatically initialized to an empty string) to implement 'on time' initialization and implicit operators to hide implementation details. The code below shows how to implement a 'double' field which is automatically initialized to Pi.

program Project44;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils;

type
  TAutoDouble = record
  private
    FValue: double;
    FInitialized: string;
    procedure Initialize(const val: double = Pi);
  public
    class operator Implicit(const rec: TAutoDouble): double;
    class operator Implicit(const val: double): TAutoDouble;
  end;

  TSomeRecord = record
    Value1: TAutoDouble;
    Value2: TAutoDouble;
  end;

{ TAutoDouble }

procedure TAutoDouble.Initialize(const val: double);
begin
  if FInitialized = '' then begin
    FInitialized := '1';
    FValue := val;
  end;
end;

class operator TAutoDouble.Implicit(const rec: TAutoDouble): double;
begin
  rec.Initialize;
  Result := rec.FValue;
end;

class operator TAutoDouble.Implicit(const val: double): TAutoDouble;
begin
  Result.Initialize(val);
end;

var
  sr, sr1: TSomeRecord;

begin
  try
    Writeln(double(sr.Value1));
    Writeln(double(sr.Value2));
    sr.Value1 := 42;
    Writeln(double(sr.Value1));
    sr1 := sr;
    Writeln(double(sr.Value1));
    Writeln(double(sr.Value2));
    Readln;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

There's, however, no nice way to make this solution more generic regarding the default value -- if you need a different default value you have to clone TAutoDouble definition/implementation and change the default value.

回答2:

AFAIK you can't without resorting to tricks that aren't worth it (maybe using interface fields which are guaranteed to be initialized).

回答3:

This would be a nice feature ... but I guess you can use a factory of some kind? or just a humble method returning a record...

来源：https://stackoverflow.com/questions/39392920/how-can-delphi-records-be-initialized-automatically

标签

Delphi