An example on how to properly use TFPSMap of FPC's fgl unit

tfpsmap.pas

{$mode objfpc}
uses sysutils, fgl;

type
  TMyKey = class
  public
    Key : String;

    constructor Create;
  end;

  TMyValue = class
  public
    Value : String;

    constructor Create;
  end;

constructor TMyKey.Create;
begin
  Key := 'A Key';
end;

constructor TMyValue.Create;
begin
  Value := 'A Value';
end;

var
  Map    : TFPSMap;
  Key    : TMyKey;
  Value  : TMyValue;
  Value2 : TMyValue;

begin
  Map   := TFPSMap.create(SizeOf(key), SizeOf(Value));
  Key   := TMyKey.create;
  Value := TMyValue.create;

  Value.Value := 'here';
  Map.add(@Key, @Value);

  Value2 := TMyValue(Map.KeyData[@Key]^);
  writeln(format('%P, %P, %S, %S', [Pointer(Value), Pointer(Value2),
                                    Value.Value, Value2.value]));
  Key.free;
  Value.Free;
  Map.free;
end.