this page accounts for some problematic areas, sketchy solutions, nonportable hacks, terrorism and other misdemeanour
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.
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
Besides, should boost or the standard library implementors eventually change the
implementation, this workaround will break.
As of this writing (1/2016), this still remains an unsolved issue.
The new standard does indeed require a comparison, but only a comparison against
nullptr, which is equivalent to the
bool conversion. It seems we need more.
|what we have here is a really evil hack, and even the notion of “equality”
is not correct. We admit that real equality can not be achieved with something
TODO investigate what we really need and come up with a better concept…
The standard allows function objects to be partially closed; this is achieved by
marking the remaining, unbound arguments in the call to
std::bind with some special
marker elements, the “argument placeholders”. These are predefined within the
standard library as
std::placeholders::_1 and consecutive, while the type of
these objects remains unspecified as by the standard. But unfortunately we need
some augmentation on top of
std::bind to help with generic partial application
of functions, i.e. we need to close systematically a sequence of arguments, both
starting from the front or from the back of the argument list. We need this,
because it is a standard functional programming technique. Consequently
our helper (
function-closure.hpp) will build placeholders on its own,
and it needs to feed placeholder types to the generated binders.
Thus we rely on the fact, that the gnu standard library implementation
has a templated type
Size of standard library facilities
Sometimes we need to know the size of an STL or Boost class, but can’t afford
to include the header and just write a
sizeof(). Because including some of those
headers incurs quite some price in terms of compilation time and even size of the
Obviously, a simple solution would be to measure those sizes and hardcode them. But what about portability? To get out of that dilemma, I created a traits class which mimics the implementation memory layout of those facilities in question, simplified as much as possible. As long as the GNU libstdc++ or Boost don’t change their implementation layout, this give us precise and fast size bounds.
When relying on that hack, we should make sure always to place some kind of
static_assert into the corresponding implementation files to ensure the real
facilites actually do fit into the guessed storage dimensions.