Could Python supplant Java?

James J. Besemer jb at cascade-sys.com
Fri Aug 23 14:18:52 CEST 2002


Roy Smith wrote:

> To make Paul's point even stronger, it not just depends on the caller,
> but it may depend on run-time events.  Imagine something like the
> following (with A and B classes as Paul described them):
>
> which version of bar() should get called is truly random and only known
> at run time.

By now you should have read my lengthy reply to Paul.

Fact of the matter, C++'s polymorphism even works in "dynamic" cases like you
cite here.  Function and Object factories actually are common in C++.

The following is valid C++:

    class A {
        int x;
        virtual int bar(){ return 1; }
    };

    class B {
        int y;
        virtual int bar(){ return 2; }
    }

    A* myFactoryFunction()
    {
        if( rand() ? 1 ){
            return new A;
        } else {
            return new B;
        }
    }

    void test()
    {
        A* ab = myFactoryFunction();
        ab->bar()    // calls A.bar() or B.bar() depending on instance
sub-type
        ab->x;        // always accesses A.x
        ab->y;        // !!compiler error -- A.y does not always exist
        delete ab;
    }

The only restriction over Python is that one of A or B would have to be
derrived from the other.  More commonly you have multiple sub-types derrived
from a common super type.  But this is usually the case if you're mixing them
together like this in the first place.  Also, bar() has to be a function, not
a variable.

I find the third use of myFactoryFunction interesting in that it produces a
compile time error in C++ while in Python it will produce a runtime error
half the time.

--jb

P.S. there is a slight bug in my previous example.  (Of course!)  Either the
classes need to be declared instead as "structs" or else an explicit
"public:" access designation needs to be added to make the fields public.

--
James J. Besemer  503-280-0838 voice
http://cascade-sys.com  503-280-0375 fax
mailto:jb at cascade-sys.com






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