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Код: Выделить всё

program Test;

{$APPTYPE CONSOLE}

type
  TMyClass = class
    x : Integer;
    procedure Test; virtual;
  end;

  PMyObject = ^TMyObject;
  TMyObject = object
    procedure Test; virtual; abstract;
  public
    x : Integer;
  end;

function Init: Pointer;
begin
  Result := TMyClass.Create;
end;

procedure TMyClass.Test;
begin
  writeln(x);
end;

var
  MyObj : PMyObject;
begin
  MyObj := Init;
  MyObj^.Test;
  MyObj^.x := 123;
  MyObj^.Test;
  readln;
end.

A class-type value is stored as a 32-bit pointer to an instance of the class, which is called an object. The internal data format of an object resembles that of a record. The object’s fields are stored in order of declaration as a sequence of contiguous variables. Fields are always aligned, corresponding to an unpacked record type. Any fields inherited from an ancestor class are stored before the new fields defined in the descendant class.
The first 4-byte field of every object is a pointer to the virtual method table (VMT) of the class. There is exactly one VMT per class (not one per object); distinct class types, no matter how similar, never share a VMT. VMTs are built automatically by the compiler, and are never directly manipulated by a program. Pointers to VMTs, which are automatically stored by constructor methods in the objects they create, are also never directly manipulated by a program.

The layout of a VMT is shown in the following table. At positive offsets, a VMT consists of a list of 32-bit method pointers—one per user-defined virtual method in the class type—in order of declaration. Each slot contains the address of the corresponding virtual method’s entry point. This layout is compatible with a C++ v-table and with COM. At negative offsets, a VMT contains a number of fields that are internal to Object Pascal’s implementation. Applications should use the methods defined in TObject to query this information, since the layout is likely to change in future implementations of Object Pascal.

Offset Type Description
–76 Pointer pointer to virtual method table (or nil)
–72 Pointer pointer to interface table (or nil)
–68 Pointer pointer to Automation information table (or nil)
–64 Pointer pointer to instance initialization table (or nil)
–60 Pointer pointer to type information table (or nil)
–56 Pointer pointer to field definition table (or nil)
–52 Pointer pointer to method definition table (or nil)
–48 Pointer pointer to dynamic method table (or nil)
–44 Pointer pointer to short string containing class name
–40 Cardinal instance size in bytes
–36 Pointer pointer to a pointer to ancestor class (or nil)
–32 Pointer pointer to entry point of SafecallException method (or nil)
–28 Pointer entry point of AfterConstruction method
–24 Pointer entry point of BeforeDestruction method
–20 Pointer entry point of Dispatch method
–16 Pointer entry point of DefaultHandler method
–12 Pointer entry point of NewInstance method
–8 Pointer entry point of FreeInstance method
–4 Pointer entry point of Destroy destructor
0 Pointer entry point of first user-defined virtual method
4 Pointer entry point of second user-defined virtual method