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From: mgates@entiat.boeing.com (Michael Gates)
Newsgroups: comp.lang.c++
Subject: Re: Multiple Inheritance Problem
Message-ID: <MGATES.91May15112133@entiat.boeing.com>
Date: 15 May 91 18:21:33 GMT
References: <74462@brunix.UUCP> <2153@godzilla.tcs.com>
Sender: nntp@bcsaic.UUCP
Organization: Boeing Computer Services, Bellevue, WA
Lines: 51
In-reply-to: robert@kohlrabi.tcs.com's message of 6 May 91 19:06:41 GMT

>In article <2153@godzilla.tcs.com> robert@kohlrabi.tcs.com (Robert Blumen) writes:
> In article <74462@brunix.UUCP>, thc@cs.brown.edu (Thomas Colthurst) writes:
> |> class Object {
> |> public:
> |>   Object();
> |>   ~Object();
> |>    virtual void Draw() = 0;
> |>    virtual void Update() = 0;
> |>    virtual Object* Dup() = 0;
> |> }
> |> class Line: public virtual Object {
> |> public:
> |>    Line();
> |>    ~Line();
> |>    void Draw();
> |> }
> |> class Dependent: public virtual Object {
> |> public:
> |>   Dependent();
> |>   ~Dependent();
> |>   void Update();
> |> }
> |> class DependentLine : public Line, public Dependent {
> |> public:
> |>   DependentLine();
> |>   ~DependentLine();
> |>   Object *Dup() { return( (new DependentLine) ); }; // error here
> |> }
> |> Sun CC version 2.1 gives me an error on the line
> |> marked above, saying
> |> "./ChildrenObjs.H", line 30: error: `new' of abstract class DependentLine
>
> For one thing, a class that inherits a pure virtual function must define it
> or redefine it to be pure virtual.

	G++ 1.39 compiles this with no trouble at all, which is
correct. For each pure virtual function, there is a path from
DependentLine to Object which defines that function. The ARM section
10.1.1 addresses this issue under the topic of name dominance. Page 205:
  
  	  A name B::f dominates a name A::f if its class B has A
  as a base. If a name dominates another no ambiguity exists
  between the two; the dominant name is used when there is a
  choice.

  (Followed by example)

Here Line::Draw dominates the Object::Draw that is also inherited
by DependentLine. Also Dependent::Update dominates Object::Update.
--
et tu mgates?