Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mitel!sce!cognos!jimp
From: jimp@cognos.uucp (Jim Patterson)
Newsgroups: comp.lang.eiffel
Subject: Re: constrained generic problem
Keywords: constrained generics, inheritance
Message-ID: <5677@cognos.UUCP>
Date: 26 Mar 89 05:14:09 GMT
References: <9091@alice.UUCP>
Reply-To: jimp@cognos.UUCP (Jim Patterson)
Organization: Cognos Inc., Ottawa, Canada
Lines: 58

In article <9091@alice.UUCP> dbl@alice.UUCP (David Levine, for Kim Bruce) writes:
>	I have been having problems with constrained generics.  
>class tree_menu[T -> SIMPLE_NODE]
>	create is
>	local	new_node : T
>	do	    new_node.create;
>The problem is that no matter how I instantiate the T, I always get the
>operations in SIMPLE_NODE on new_node.

The problem is that, in the tree_menu class of your example the
class-type which T is instantiated with isn't really known so in fact
new_node is always created as a SIMPLE_NODE (as you have described).

We have used the following workaround; an instance of the class to
which T is bound is created, and passed to any procedures which need
to create the complex type. Then, that instance can be used to clone a
new instance of that type.  In your example, this would be as follows:

   create(model : T) is
   local new_node : T;
   do 
     new_node.clone(model);
   end;
 -- in client
   a : COMPLEX_NODE;
   t : TREE_MENU[COMPLEX_NODE];
   a.create;
   t.create(a);

>P.S.  Is anyone else having the problem of getting '5's all over the place
>when trying to use the interactive debugger?

We had this problem; it seems to be related to a failure by the viewer 
to recognize timing data in the TERMCAP entries used by Curses. If you
remove this information from the TERMCAP entry for your terminal, it
should go away. (You may actually need the time delay however, in which
case you will have another problem). For example, your vt100 entry likely
looks something like this (I've left most of it out):

d1|vt100|dec vt100:\
	:am:co#80:li#24:cl=50\E[;H\E[2J:bs:cm=5\E[%i%2;%2H:nd=2\E[C:up=2\E[A:\
	:ce=3\E[K:cd=50\E[J:so=\E[7m:se=\E[m:us=\E[4m:ue=\E[m

The timing information is any numeric data immediately following an equal
sign (=), so with this information stripped the entry will look as follows.

d1|vt100|dec vt100:\
	:am:co#80:li#24:cl=\E[;H\E[2J:bs:cm=\E[%i%2;%2H:nd=\E[C:up=\E[A:\
	:ce=\E[K:cd=\E[J:so=\E[7m:se=\E[m:us=\E[4m:ue=\E[m

This problem might be fixed in the next Eiffel release; I really
don't know.

-- 
Jim Patterson                              Cognos Incorporated
UUCP:decvax!utzoo!dciem!nrcaer!cognos!jimp P.O. BOX 9707    
PHONE:(613)738-1440                        3755 Riverside Drive
                                           Ottawa, Ont  K1G 3Z4