File : terminal_interface-curses-forms-field_types-user.adb 
with System.Address_To_Access_Conversions;
with Terminal_Interface.Curses.Aux; use Terminal_Interface.Curses.Aux;
package body Terminal_Interface.Curses.Forms.Field_Types.User is
   procedure Set_Field_Type (Fld : Field;
                             Typ : User_Defined_Field_Type)
   is
      function Allocate_Arg (T : User_Defined_Field_Type'Class)
                             return Argument_Access;
      function Set_Fld_Type (F    : Field := Fld;
                             Cft  : C_Field_Type := C_Generic_Type;
                             Arg1 : Argument_Access)
                             return Eti_Error;
      pragma Import (C, Set_Fld_Type, "set_field_type_user");
      function Allocate_Arg (T : User_Defined_Field_Type'Class)
                             return Argument_Access
      is
         Ptr : constant Field_Type_Access
             := new User_Defined_Field_Type'Class'(T);
      begin
         return new Argument'(Usr => System.Null_Address,
                              Typ => Ptr,
                              Cft => Null_Field_Type);
      end Allocate_Arg;
   begin
      Eti_Exception (Set_Fld_Type (Arg1 => Allocate_Arg (Typ)));
   end Set_Field_Type;
   package Argument_Conversions is
      new System.Address_To_Access_Conversions (Argument);
   function Generic_Field_Check (Fld : Field;
                                 Usr : System.Address) return Curses_Bool
   is
      Result : Boolean;
      Udf    : constant User_Defined_Field_Type_Access :=
        User_Defined_Field_Type_Access
          (Argument_Access (Argument_Conversions.To_Pointer (Usr)).all.Typ);
   begin
      Result := Field_Check (Fld, Udf.all);
      return Curses_Bool (Boolean'Pos (Result));
   end Generic_Field_Check;
   function Generic_Char_Check (Ch  : C_Int;
                                Usr : System.Address) return Curses_Bool
   is
      Result : Boolean;
      Udf    : constant User_Defined_Field_Type_Access :=
        User_Defined_Field_Type_Access
          (Argument_Access (Argument_Conversions.To_Pointer (Usr)).all.Typ);
   begin
      Result := Character_Check (Character'Val (Ch), Udf.all);
      return Curses_Bool (Boolean'Pos (Result));
   end Generic_Char_Check;
   
   
   function C_Generic_Type return C_Field_Type
   is
      Res : Eti_Error;
      T   : C_Field_Type;
   begin
      if M_Generic_Type = Null_Field_Type then
         T := New_Fieldtype (Generic_Field_Check'Access,
                             Generic_Char_Check'Access);
         if T = Null_Field_Type then
            raise Form_Exception;
         else
            Res := Set_Fieldtype_Arg (T,
                                      Make_Arg'Access,
                                      Copy_Arg'Access,
                                      Free_Arg'Access);
            Eti_Exception (Res);
         end if;
         M_Generic_Type := T;
      end if;
      pragma Assert (M_Generic_Type /= Null_Field_Type);
      return M_Generic_Type;
   end C_Generic_Type;
end Terminal_Interface.Curses.Forms.Field_Types.User;