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Date: Tue, 30 Jul 2002 11:30:50 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Newsgroups: comp.lang.ada
Subject: Re: Multiple interface inheritance workaround in Ada 95 / Ada 0x
 to satisfy a Java language advocate?
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Date: 2002-07-30T11:30:50-04:00
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Jean-Pierre Rosen wrote:
> Yes. Or more exactly, the design pattern provides functions
 > that allow you to get an I1 or I2 view for a C object, and to
 > go back from an I1 or I2 view back to the original C object.
> 
> And you are correct that this is not obvious to achieve.
 > That's what prompted me to make a paper of it!

Going back to the original object isn't quite the same thing.
It's something of a red herring since this kind of thing is
extremely rarely used, but you can't claim full conformity
if you don't support it.

Here is what this kind of code looks like:

// Java
void f(I1 i1) { if (i1 instanceof I2) { I2 i2 = (I2)i1; } }

// C++
void f(I1 *i1) { I2 *i2 = dynamic_cast<I2 *>(i1); if (i2) { /*...*/ } }

As long as the underlying object has an accessible i2 in its
inheritance graph, the conversion works, and the code does not
need to know what the common ancestor class is, and will work
for many different ancestor types.

This is accomplished by compiler magic. The compiler has access
to the virtual tables and can search them for the presence of
appropriate bases.