Inheritance with Ada types
in [ADA]
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From: Bryan on 8 Feb 2010 21:16 I'm new to the OOP features of Ada and I'm trying to wrap my head tagged types. I've been reading the somewhat abstruse "Ada for Software Engineers" and while I understand the examples in the text, I would like to experiment on my own to further my understand. I'm trying to figure out how I can correctly--pardon my C++--"subclass" a base class to create a new derived "class". I created two simple packges as follows: parent.ads: with Ada.Strings.Unbounded; use Ada.Strings.Unbounded; package Parent is type Parent is abstract tagged private; private type Parent is abstract tagged record Name: Unbounded_String; end record; end Parent; derived.ads: with Parent; use Parent; package Derived is type Derived is new Parent with private; private type Derived is new Parent with record Sub_Name: Unbounded_String; end record; end Derived; I can compile parent.ads and the object file and ALI file are produced. I'm unable to get derived.ads to compile, however. I get the following error: $ gnatmake -c parent derived.ads gcc -c derived.ads derived.ads:3:23: subtype mark required in this context derived.ads:3:23: found "Parent" declared at parent.ads:2 derived.ads:5:23: subtype mark required in this context derived.ads:5:23: found "Parent" declared at parent.ads:2 gnatmake: "derived.ads" compilation error If I make the Derived type in a child package of the parent package, I can get both interfaces to compile. Perhaps I am misunderstanding something about Ada OOP, but is it possible to have a derived package that is not a child package of the base? Must I use a child package, or keep the derived type in the same package as the base type?
From: Hibou57 (Yannick Duchêne) on 8 Feb 2010 21:57 On 9 fév, 03:16, Bryan <brobinson....(a)gmail.com> wrote: > I created two simple > packges as follows: > > parent.ads: > > with Ada.Strings.Unbounded; use Ada.Strings.Unbounded; > package Parent is > type Parent is abstract tagged private; > private > type Parent is abstract tagged > record > Name: Unbounded_String; > end record; > end Parent; > > derived.ads: > > with Parent; use Parent; > package Derived is > type Derived is new Parent with private; > private > type Derived is new Parent with > record > Sub_Name: Unbounded_String; > end record; > end Derived; Oops : package and type name share the same name and you there are use clause... the use clause is particularly evil here. > $ gnatmake -c parent derived.ads > gcc -c derived.ads > derived.ads:3:23: subtype mark required in this context > derived.ads:3:23: found "Parent" declared at parent.ads:2 The message refer to > package Parent is > derived.ads:5:23: subtype mark required in this context > derived.ads:5:23: found "Parent" declared at parent.ads:2 The same as above Either rename your packages (like Parents and Deriveds ... although the latter is not correct English) or drop the use clause and refer to your Parent ancestor as Parent.Parent instead. By the way, the With clause for Ada.Strings.Unbounded is missing in Derived.ads
From: Jean-Pierre Rosen on 9 Feb 2010 04:11 Hibou57 (Yannick Duch�ne) a �crit : > On 9 f�v, 03:16, Bryan <brobinson....(a)gmail.com> wrote: > Oops : package and type name share the same name and you there are use > clause... the use clause is particularly evil here. Why? The use clause has no effect in this case! >> $ gnatmake -c parent derived.ads >> gcc -c derived.ads >> derived.ads:3:23: subtype mark required in this context >> derived.ads:3:23: found "Parent" declared at parent.ads:2 > The message refer to >> package Parent is > >> derived.ads:5:23: subtype mark required in this context >> derived.ads:5:23: found "Parent" declared at parent.ads:2 > The same as above > > Either rename your packages (like Parents and Deriveds ... although > the latter is not correct English) or use the convention of giving the class name to the package, and some fixed name like "object" or "instance" to the type. Full rationale for this notation is available in my paper "A naming convention for classes in Ada 9X", downloadable from http://www.adalog.fr/publica2.htm > or drop the use clause and refer to > your Parent ancestor as Parent.Parent instead. Of course, you don't need to drop the use clause to write Parent.Parent! I don't understand why people think that as soon as you have a use clause, you cannot use full names. If within the scope of a use clause you don't get what you want, or if you find that in some cases full names are more readable, by all mean, use full notation! -- --------------------------------------------------------- J-P. Rosen (rosen(a)adalog.fr) Visit Adalog's web site at http://www.adalog.fr
From: Hibou57 (Yannick Duchêne) on 9 Feb 2010 05:19 On 9 fév, 10:11, Jean-Pierre Rosen <ro...(a)adalog.fr> wrote: > > clause... the use clause is particularly evil here. With package named Parent, containing a type named Parent, having a use clause on Parent brings into a context where the name Package can be both resolved as a package or as a type. The use clause is clearly involved here, as one the way to get ride of this error is to remove the use clause. The other way being to rename either the type either the package. May be I was wrong to say the use clause is particularly evil there : I should have said the use clause is particularly vicious there (due to its vicious side effect in such a kind of context).
From: Hibou57 (Yannick Duchêne) on 9 Feb 2010 05:22 On 9 fév, 10:11, Jean-Pierre Rosen <ro...(a)adalog.fr> wrote: > I don't understand why people think that as soon as you have a use > clause, you cannot use full names. If within the scope of a use clause > you don't get what you want, or if you find that in some cases full > names are more readable, by all mean, use full notation! Agree with this. True that having a Use clause does not implies the prefix notation is not available anymore. Well, let say in this context he should not rely on the use clause to refer to the type Parent.
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