What “type of” declaration represents in Delphi and how can it be used

Question

There is some strange code in Datasnap.DSReflect unit

  TDSAdapterClassType = type of TDSAdapterClass;

  TDSAdapterClass = class(TPersistent)
  pri         


        

Answer 1

It seems to be related to PTypeInfo based in the TypeKind as you can write this:

program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  TIntType = type of Integer;
  TInt64Type = type of Int64;

var
  intType: TIntType;
  int64Type: TInt64Type;
begin
  try
    intType := Integer;
    Assert(Pointer(intType) = TypeInfo(Integer));
    intType := Cardinal;
    Assert(Pointer(intType) = TypeInfo(Cardinal));
    intType := NativeInt;
    Assert(Pointer(intType) = TypeInfo(NativeInt));
    int64Type := Int64;
    Assert(Pointer(int64Type) = TypeInfo(Int64));
    int64Type := UInt64;
    Assert(Pointer(int64Type) = TypeInfo(UInt64));
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;
end.

But it does not work properly with all types and throws internal compiler errors for some.