Re: [GD-General] Re: Simulating constrained genericity in C++

[Patrick M D. <pa...@wa...>]

On Thu, 27 Dec 2001, Joe wrote:

> So assuming point2d was an interface too, wouldn't you
> just do
> interface point3d extends point2d{
> public float z();
> }
> and then your old implimentation should work
>

Here is a more compelling example directly from the JDK.

The interface AbstractCollection contains a number of methods that require
more functionality than is actually required:


    public boolean containsAll(Collection c)
    public boolean addAll(Collection c)
    public boolean removeAll(Collection c)
    public boolean retainAll(Collection c)

Each of these could be implemented on a class that implemented a single
method (e.g. 'contains' or 'iterator').  There was a proposal to
introduce additional interfaces like so:

    public boolean containsAll(Container c)
    public boolean addAll(Iterable c)
    public boolean removeAll(Container c)
    public boolean retainAll(Container c)

But this was rejected because it increased the number of interfaces that
were required.

There's nothing the end-programmer can really do about this without
breaking compatibility with the standard libraries.  This kind of problem
is pervasive throughout languages like Java and C++ that do not
separate the notions of inheritance and subtyping.  I suspect that you
could find similar problems in Eiffel libraries as well.

Patrick