Re: Initializers

[Georg Bauhaus <bauhaus@arcor.de>]

On Tue, 2006-12-19 at 18:02 -0800, markww wrote:
> Hi everyone,
> 
> I'm trying to compare C++ to Ada. I'm looking specifically at Ada's
> lack of 'initializers'. I am guessing that this means there is no
> 'constructor' to Ada packages maybe?

Others have given examples of initialization of record components
and of package initialization blocks. (Two different things.)
Another 2c:

There are also construction subprograms and nested Factory packages
that are commonly used for initializing objects. Some of this is
explained in the Ada wikibook, and in many Ada text books.

In addition, consider:

- types whose objects must not be initialised, not even default
  initialized. A reason might be that their bits are mapped
  to hardware. Initialization would mean sending signals to the
  hardware. You don't want some signals sent only because the
  language uses some default initialization. So in Ada, you can
  turn initialization off, entirely.

- limited types. For every object of a limited record you can
  obtain its access within the record definition. This allows
  one or more construction functions invoked automatically
  when an object is created but not initialized "by hand":

  package P is

    type L is limited private;

    package Factory is

      function make(item: access L) return Natural;

    end Factory;

  private

    type L is limited record
      hook: Natural := Factory.make(L'access);
      stuff: Integer;
    end record;

  end P;

  Factory.make has access to all the components of its Item
  parameter, including Stuff. It is invoked automatically
  for an object of type L that isn't explicitly initialized.

- discriminated types. These will force partial initialization
  because the discriminant needs to have a value to make the
  object be of some specific type, and have a known size etc.
  There are some tricks here.

- nested scope. This is useful if you compare initialization in
  C++ and Ada because nesting is built into Ada and can be used
  here. Support for nested scopes (dynamic + static) is limited
  in C++ by comparison I think. There are cases where
  you would pass parameters to a C++ constructor, where in Ada
  you needn't pass them because they are in (local) scope.


 -- Georg