Node:SetType, Next:shl, Previous:SetLength, Up:Reference
procedure SetType (var SomeObject; VMT: PObjectType);
The procedure SetType explicitly assigns a value to the
implicit VMT field of an object. This is normally done implicitly
when a constructor is called.
You can use this to write a polymorphic I/O routine which reads an
object from a file. In this case, you cannot reasonably use
New to allocate the storage, but you GetMem it and
initialize the object manually using SetType before calling
the constructor explicitly.
The only values you should assign to an object via SetType
are actual VMT pointers that were obtained via TypeOf. In
particular, declaring a record like the one shown in the description
of PObjectType and assigning a pointer to it to an object via
SetType will usually not work because the virtual method
pointers are missing.
Since SetType is a dangerous feature, it yields a warning
unless {$X+} is given.
SetType is a GNU Pascal extension.
program SetTypeDemo;
type
BasePtr = ^BaseObj;
BaseObj = object
constructor Load;
end;
ChildObj = object (BaseObj)
constructor Load;
end;
constructor BaseObj.Load;
begin
end;
constructor ChildObj.Load;
begin
end;
{$X+}
{ This is somewhat fragmentary code. }
function GetObject (var InputFile: File) = Result: BasePtr;
const
VMTTable: array [1 .. 2] of PObjectType =
(TypeOf (BaseObj), TypeOf (ChildObj));
var
Size: Cardinal;
TypeID: Integer;
VMT: PObjectType;
begin
{ Read the size of the object from some file and store it in `Size'. }
BlockRead (InputFile, Size, SizeOf (Size));
{ Allocate memory for the object. }
GetMem (Result, Size);
{ Read some ID from some file. }
BlockRead (InputFile, TypeID, SizeOf (TypeID));
{ Look up the `VMT' from some table. }
{ Range checking wouldn't be a bad idea here ... }
VMT := VMTTable[TypeID];
SetType (Result^, VMT);
{ Now the object is ready, and the constructor can be called. }
{ Look up the correct constructor from some table and call it. }
end;
begin
end.