Re: 'Base

[Matthew Heaney <matthewjheaney@earthlink.net>]

ada_student@yahoo.com writes:

> Indefinite subtypes cannot be allowed under any circumstances.

I don't understand.  What is type String?


> It sounds like a hack just inorder to allow one class of array object
> declarations into the language.

I would hardly call being able to declare strings of different lengths a hack!


> How can a statically typed language allow a type whose size is not known at
> compile time?

There's a "dope vector" associated with string objects, that describes the
length and bounds of the string.


> The Java array type certainly wins over Ada in this regard.

But array objects in Ada can be declared on the stack.  This allows you do
things like:

  procedure Op1 (N : Natural) is
    S : String (1 .. N);
  begin

  procedure Op2 (S : String) is
    S2 : String := S;
  begin

In neither case is heap necessary.  You can do this sort of thing with tagged
types too:

  procedure Op (O : T'Class) is
    O2 : T'Class := O;
  begin


One nice feature is that if you do allocate an object whose type is class-wide,
then you can initialize the object the same as in the example above:

  type T_Class_Access is access T'Class;

  procedure Op2 (O : T'Class) is
    O2 : constant T_Class_Access := new T'Class'(O);
  begin

This is built into the language; you don't need a dispatching clone method.