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Xref: reader02.eternal-september.org comp.lang.ada:52919
Date: 2018-06-04T03:29:24-07:00
List-Id: <comp.lang.ada>

Dear.all,=20
I have a doubt about what it could be the best solution for some code that =
I am writing.  I am afraid that the introduction before the actual question=
 is a bit long. =20

** The problem **

Let me use an example.  Suppose I am writing a program to plot, say, a GANT=
T chart.  I want the user to be able to choose the output format (say, PDF,=
 SVG or even LaTeX) by giving a suitable option on the command line and I w=
ant to be able to add new output format easily, maybe even load the "format=
 handler" as a dynamic library at run-time (so I do not need recompilation)=
.  (Incidentally, I did some experiments with the dynamic loading and it wo=
rks)

The structure I give to the code is as follows: first I define an interface=
 for an "abstract plotter," for example

  package Plotters is=20
    type Abstract_Plotter is interface;

    procedure Line(P: Abstract_Plotter;
                   X1,Y1, X2, Y2:Float)=20
    is abstract;

    -- And so on...
  end Plotters;

Every actual plotter will be a descendant of Abstract_Plotters, for example

  package Plotters.SVG is
     type SVG_Plotter is=20
     new Abstract_Plotter with private;

     -- Blah, blah ...
  end Plotters.SVG;

In order to allow the user to select the actual plotter via the command lin=
e I use a generic package Plugin_Table that implements a kind of "map" from=
 names to descendant of Abstract_Plotter.  Plugin_Table provides two operat=
ions:

  (1) Register_Plugin(T: Tag; Name: String) =20

         used by a concrete plotter to "register" itself to the table.  Usu=
ally it is called from the "initialization" part of the body of the package=
 definynig the concrete plotter.  For example, plotters-svg.adb could call

     Register_Plugin(SVG_Plotter'Tag, "svg");

  (2) Get_Plugin (Name:String) return Abstract_Plotter'Class

       to be called to generate a concrete plotter.

In a future version Plugin_Table will be able to search for some dynamic li=
brary to be loaded if a plugin with the given name is not registered.

   ** The Question **=20

In order to have all the built-in plugins registered in my executable, I ne=
ed to "with" them, so that they are linked to my code and the initializatio=
n part of their bodies is elaborated. However, no part of my code call expl=
icitly, say, SVG_Plotter.  Therefore, my solution is to add something like

  with Plotters.SVG;
  pragma Warnings(Off, Plotter.SVG);

in the "main." It works, but it leaves me a bit unsatisfied since it seems =
an "hack".  Also, since no actual code is called directly I am not sure if =
the binder could take the liberty of not including them (maybe it cannot, b=
ut I am not sure).

Do you have any better solution?

Also, I am not sure if I risk getting in trouble with Elaboration order.  I=
 think not, but I am not 100% sure.  What do you think?


Thanks,

Riccardo