Unable to declare string property defaults

Question

I'm writing a component which consists of many properties which are to appear in the Delphi IDE Object Inspector (published properties)...

type
  TMyComponent = class(TComponent)
  private
    FMyProperty: String;
  published
    property MyProperty: String read FMyProperty write SetMyProperty default 'Something';
  end;

However, it's not allowing me to apply a default value to a string property...

[DCC Error] MyUnit.pas(278): E2146 Default values must be of ordinal, pointer or small set type

All other property defaults work fine (Integer, Enum, etc.).

My goal is to A) not save string properties to the DFM if they're the default value, and B) show the value in the Object Inspector as Bold if it's not the default, and regular if it is. There are over 130 properties which show for this component, and about 50 of them are string properties, some with rather large default values.

Why am I not allowed to declare a string property with a default value? Is this a shortcoming with Delphi, or is there a technical reason why strings can't be defaulted?

EDIT

If you really want to know what I'm doing, I'm encapsulating Inno Setup and wrapping the functionality into a component with an extensive property/collection editor. This topic pertains to just the Setup section alone, which consists of actually over 100 properties. Only about 20 of these properties are expected to actually be used for simple implementation, and therefore I don't want all the rest of those string properties to bloat the size of the DFM (if they're set to their defaults). Based on how the component is set up, it will produce an Inno Setup script file.

@TLama I see that, but why? Strings are so common, why would the compiler skip such a common type as a string? — Jerry Dodge
That's the question merely targeted to the compiler designers. Personally I don't see anything preventing implementation of all the types from my point of view. In a similar manner you can ask, why the values given by the default specifiers are not used to initialize property values. — TLama
@TLama I am initializing all the defaults (of over 100 properties) in the constructor. That wasn't relevant to the question though, so I didn't include it. — Jerry Dodge
But back to your problem. Simply make a stored directive method which will return False when the property field will equal to the default value. ...as Remy shown in his answer... — TLama

Remy Lebeau Remy Lebeau · Accepted Answer · 2014-02-23T01:38:12

Only numeric properties can have a default value specified in the property declaration. However, you can use the stored specifier instead, eg:

type
  TMyComponent = class(TComponent)
  private
    FMyProperty: String;
    function MyPropertyIsStored: Boolean;
    procedure SetMyProperty(const Value: String);
  public
    constructor Create(AOwner: TComponent); override;
  published
    property MyProperty: String read FMyProperty write SetMyProperty stored MyPropertyIsStored;
end;

constructor TMyComponent.Create(AOwner: TComponent);
begin
  Inherited;
  FMyProperty := 'my default value';
end;

function TMyComponent.MyPropertyIsStored: Boolean;
begin
  Result := FMyProperty <> 'my default value';
end;

procedure TMyComponent.SetMyProperty(const Value: String);
begin
  if FMyProperty <> Value then
  begin
    FMyProperty := Value;
    // update component as needed...
  end;
end;

Unable to declare string property defaults

2 Answers