Hi all,

I have two questions about generics. I am writing a two-dimensional table class. Say:

type
  generic TTable<TValue> = class
  ... ...
  public
    constructor Create(ADefaultValue: TValue);
  end;

My questions are:

1) if I am not writing a generic class I can define default value for the constructor, i.e.

constructor Create(ADefaultValue: Double = 0);

for generic classes, how can I define a default value?

2) if I cannot define a default value, I hope to do so while specializing it. i.e.

type
  TNumericTable = specialize TTable<Double>
  public
    constructor Create(ADefaultValue: Double = 0); override;
  end;

That is, can I "inherit" a generic class while specializing it?

Thanks,
Shannon
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Thanks Sven. In the case I use Default() does it mean it always has fixed defaults with specific types, e.g. 0 for integer '' for string. Or I can set my own default value?

2013/2/22 Sven Barth

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You can use the new intrinsic Default(TypeName) which is available in FPC 2.7.1. Your declaration would look like this:

=== snippet begin ===

constructor Create(ADefaultValue: TValue = Default(TValue));

=== snippet end ===

You need to pay attention though with what you specialize TTable. You can't use a record for example, because records can not have default values in parameters (this has nothing to do with generics or the Default() intrinsic). Classes and interfaces are ok they as are all primitive types.

As a sidenote: Default() can not only be used in constructor parameter lists, but also for normal functions and in normal code. Using it in const sections or var sections does not yet work in every case (primitive types work, classes, interfaces, records and objects do not).

You would need to write it this way:

=== code begin ===

type
   TNumericTable = class(specialize TTable<Double>)
   public
     constructor Create(ADefaultValue: Double = 0);
   end;
constructor TNumericTable.Create(ADefaultValue: Double);
begin
   inherited Create(ADefaultValue);
end;

=== code end ===

Regards,
Sven

Xiangrong Fang wrote

Thanks Sven. In the case I use Default() does it mean it always has fixed defaults with specific types, e.g. 0 for integer '' for string. Or I can set my own default value?

It is a fixed default value. Simply spoken it's always zero. Even in cases like the following:

=== example begin ===

type
   TTest = record
     case Boolean of
       True: (a: 5..10);
       False: (b: -10..-5);
   end;
var
   t: TTest;
begin
   t := Default(TTest);
   Writeln(t.a); // 0
   Writeln(t.b); // 0
end.

=== example end ===

Regards,
Sven