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From: "G.B." <bauhaus@futureapps.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Instantiating package problems
Date: Mon, 4 Jan 2016 14:43:20 +0100
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Xref: news.eternal-september.org comp.lang.ada:29006
Date: 2016-01-04T14:43:20+01:00
List-Id: <comp.lang.ada>

On 04.01.16 01:30, Andrew Shvets wrote:
> Ok, I think I understand.  In Ada, every vanilla package imports all of the public functions/procedures to be used and can be thought of as static functions in C++.  However, I can create instances of different records in order to retain an element of state like I would in C++ when I create an object.
>

Consider this, just for some wording.

with Energy, Physics;

package Named_Things is

    function Consumption return Physics.Mass;
    -- equivalent of total energy transformed by traffic.

    type Car is private;

    procedure Method_1
      (The_Object : in out Car;
       Amount     : in     Energy.Fuel);

    function Method_2
      (The_Object : in Car)
      return Physics.Velocity;

    type Roadkill is
      (Squirrels, Cats, Rat, Opossums, Raccoons, Dogs, Deer);

    procedure Hit (Object_A : Roadkill; Object_B : Car);
    --  adds to goods consumed by traffic.

private

    Total_Consumption : Physics.Litres;
    --  burned by all cars, cf. Method_1, also equiv. Hit

    type Car is record
       Fill  : Energy.Fuel;
       -- ...
    end record;

end Named_Things;

Subprograms Method_1 and Method_2 belong to the primitive operations
of type Car, declared next to it as primitive subprograms. All three,
the type Car and its primitive subprograms, are declared in package
Named_Things.
Creating an object of type Car,

   My_Car : Named_Things.Car;

gives an instance of the type Car, for state;
calling Method_1 and passing this instance as the first argument
makes Method_1 operate on this object (formally, on the type, I think);
additionally, the body of Method_1 may assign a new value to the
variable Named_Things.Total_Consumption.

However, the package also has the declaration of another type
and two more procedures. One of them is referring to both types
in its parameter profile, viz. Hit.