Re: Design By Contract

[Joachim Durchholz <joachim.durchholz@munich.netsurf.de>]

Patrick Doyle wrote:
> >Dynamic binding and "frozen" are orthogonal issues.
> 
>   Now, have a look at the quote you took, and tell me how it
> contradicts this.

Nonvirtual C++ functions and frozen Eiffel routines are not the same.

1) Even a frozen routine can be polymorphic - it may itself override an
inherited routine. (This is not an issue for the frozen features
declared in ANY, PLATFORM or GENERAL.)
2) There is no way in Eiffel to achieve the effect of C++ nonvirtual
member functions. If you have a routine
  blah (x: A_CLASS) is
  do
    x.do_something
  end
the standard C++ policy (nonvirtual routine) would be to always call
A_CLASS.do_something, regardless of wether the parameter passed in for x
is of that type or of a descendant. In Eiffel, the decision which
routine to call will always be based on the run-time type of x, *never*
on the declared type along.
(Of course, if do_something is frozen in A_CLASS, no polymorphism is
possible, so the compiler should optimize this to a static call. But
that's the same as when the compiler determines that only A_CLASS
objects will be passed for x in the given program.)

Regards,
Joachim
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