Re: derived formal types and known discriminants

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 04/07/2017 02:30, sbelmont700@gmail.com wrote:

> At the risk of having some of the other thread's shade thrown in
> here, aren't tagged discriminated records the de facto way of doing
> initialization?  That is, extending a Controlled type and then using
> discriminants in place of ctor arguments?
Sure. I am using this quite frequently. Sometimes I go this way:

    type Factory_Type is record ...;
    type Actual_Type (Parameters : not null access Factory_Type) is
      tagged ...;

There are obvious limitations to that. Especially that you have to keep 
parameters after object construction. Of course one can also do this:

    type Factory_Type is record ...;
    type Factory_Type_Ptr is access Factory_Type'Class;
    type Actual_Type (Parameters : Factory_Type_Ptr) is
      new Limited_Controlled ...;

Then in the Initialize you call Unchecked_Deallocate on Parameters and 
comment with an appropriate number of exclamation marks that objects 
must be created this way:

    X : Actual_Type (new Parameter'(...));

Isn't getting rid of such constructs a valid argument to have proper 
constructors?

BTW, if discriminants were abstracted as they should have been, that 
would be the way to implement constructors with parameters. You could 
declare a parameter as a public discriminant non-existing in the actual 
implementation and the type user would have to supply it when declaring 
an object.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de