Re: Ada Annoyances

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

On 2017-06-24 15:06, pythoner6@gmail.com wrote:

> As I understand it, the penalty in Ada is going to some extra space 
> for the tag (how big is this? I haven't found much info on the size
> of a tag, which I'm assuming is because this may be implementation
> dependent?),
Do this:

    type Empty is tagged null record;

Empty'Size is the size in bits.

> and the smart pointer will get passed around by reference, as
> opposed to by value which would make more sense. While for most part
> I'm not too  concerned about the extra space required for the tag (in other cases),
> for a smart pointer that's probably about doubling the size which is
> unfortunate.

That depends. Pointers to reference-counted objects are much better to 
passed by reference. Because by-copy passing will require incrementing 
and decrementing the reference count, which in concurrent environment 
requires expensive locking.

The sorts of pointers usually passed by copy are normally superfluous in 
Ada.

> Method calls won't be dynamically dispatched unless you're dealing
> with a Smart_Pointer'Class and calling methods that take a Smart_Pointer
> (which I don't see much reason to do if you want a low overhead smart
> pointer), right?

Wrong. In Ada a pointer need not to be tagged in order to dispatch on 
the target.

    type T is tagged ...
    procedure Foo (X : in out T);
    type S is new T with ...
    type Pointer is access T'Class;

    X : Pointer := new S;

    X.Foo -- This dispatches

>> In C++ when the object of derived class is constructed, its base
>> class constructor is called first and at that time the object is
>> considered to have the Base dynamic type
[...]
>> In this sense and this particular context, C++ offers better and
>> stronger (!) type safety than Ada.
> Ah, I see.

An object may not change its type. When not constructed it simply does 
not exist.

C++ construction model is safer where it supports user-defined hooks for 
specific types.

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