Re: Inheritance and Polymorphism in Ada !!

[tmoran@bix.com]

> 1  procedure inheritance_polymorphism is
> 2
> 3  type type_a is tagged record
> 4           first_field : integer;
> 5       end record;
> 6
> 7  type type_b is new type_a with record
> 8           second_field : integer;
> 9       end record;
> 10
> 11 type pointer_type is access all type_a'class;
> 12 pointer      : pointer_type;
> 13
> 14 begin
> 15     pointer := new type_b;
> 16a    type_b(pointer.all).second_field := 1;
> 16b    pointer.second_field := 1;
> 17 end inheritance_polymorphism;
> See line 16, the type of the referenced object must be known in advance.
> With "pointer.second_field :=1" the compiler fails. It seems that the
> strong typing restricts the features of polyormism in Ada. Any comments?

  Suppose you insert, just after line 15,
    if phase_of_moon = full then pointer := new type_a;end if;
Then at run time the code would have to check that whatever "pointer"
was currently pointing to, contained a field named "second_field".
Further, since perhaps type_b's second_field might not be integer, eg
     type type_b is new type_a with record
              second_field : character;
          end record;
it would have to check that the particular "second_field" was an
integer before it could assign a "1" to it.

  Suppose "pointer.second_field := 1;" is in a subroutine called from
someplace where "pointer" points to some type_d, which might or might
not have a field named "second_field", which might or might not be an
integer.  This isn't known at the time of compiling
"pointer.second_field := 1;" so the generated code would need to call
some routine associated with the tag on pointer.all and say "I'd like
to store an integer with value 1 in a field named "second_field'.
Please either do that or raise some appropriate exception."

  Would you really want all that?

> See line 16, the type of the referenced object must be known in advance.
  No.  If you change line 11 to
> 11 type pointer_type is access all type_b'class;
  then "pointer.second_field :=1;" compiles just fine, since it's
clear that anything the pointer might point to, does indeed have
an integer "second_field".

> With "pointer.second_field :=1" the compiler fails.
 No, the compiler succeeds in detecting an error - before run time.