Dark Corners
============

_this page accounts for some problematic areas, sketchy solutions,
nonportable hacks, terrorism and other misdemeanour_

Library
-------

Equality of Functors
~~~~~~~~~~~~~~~~~~~~
One of the more important recent additions to the C++ language are function objects.
In addition to the features actually provided by the boost implementation, the tr1 report
also requires function instances to implement an equality operator. Unfortunately the
implementation approach choosen by boost makes a 100% correct implementation of
comparision very dificult, if not impossible. Thus, the boost developers refused
to implement this feature.

The bad news is that really using the power of opaque function objects quickly drove
us (Lumiera) into a situation where such an equalty test and a hash calculation on
function objects would be necessary. The whole point of using function objects is
the ability to ``erase'' specific details, which has the downside that the resulting
generic objects are opaque and often dificult to manage, when it comes to storing
and retrieving objects building on such functors.

Thus I built an hack, based on the implementation details of boost::function.
In +functor-util.hpp+ we define a +class HijackedFunction+, which has the same
data layout as the original boost::function. After forcibly casting such an function
(reference or pointer) into a +HijackedFunction+, we're able to inspect and evaluate
the implementation pointers for equality comparison and hash value calculation.
This approach works and actually detects copied functions to be _equal_, but is
unable to pinpoint _equivalence_, e.g. functors bound to the same function with
the same arguments through separate but otherwise identical invocations of +bind+.
Besides, should boost or the standard library implementors eventually change the
implementation, this workaround will break.