The situation focussed by this concept is when an API needs to expose a sequence of results, values or objects, instead of just yielding a function result value. As the naive solution of passing an pointer or array creates coupling to internals, it was superseded by the GoF Iterator pattern. Iteration can be implemented by convention, polymorphically or by generic programming; we use the latter approach.
Lumiera Forward Iterator concept
An Iterator is a self-contained token value, representing the promise to pull a sequence of data
rather then deriving from an specific interface, anything behaving appropriately is a Lumiera Forward Iterator.
the client finds a typedef at a suitable, nearby location. Objects of this type can be created, copied and compared.
any Lumiera forward iterator can be in exhausted (invalid) state, which can be checked by
especially, default constructed iterators are fixed to that state. Non-exhausted iterators may only be obtained by API call.
the exhausted state is final and can’t be reset, meaning that any iterator is a disposable one-way-off object.
when an iterator is not in the exhausted state, it may be dereferenced (
*i), yielding the “current” value
moreover, iterators may be incremented (
++i) until exhaustion.
The Lumiera Forward Iterator concept is a blend of the STL iterators and iterator concepts found in Java, C#, Python and Ruby. The chosen syntax should look familiar to C++ programmers and indeed is compatible to STL containers and ranges. To the contrary, while a STL iterator can be thought off as being just a disguised pointer, the semantics of Lumiera Forward Iterators is deliberately reduced to a single, one-way-off forward iteration, they can’t be reset, manipulated by any arithmetic, and the result of assigning to an dereferenced iterator is unspecified, as is the meaning of post-increment and stored copies in general. You should not think of an iterator as denoting a position — just a one-way off promise to yield data.
Another notable difference to the STL iterators is the default ctor and the
bool conversion. The latter allows using iterators painlessly within
while loops; a default constructed iterator is equivalent to the STL
end() value — indeed any container-like object exposing
Lumiera Forward Iteration is encouraged to provide such an
additionally enabling iteration by
std::for_each (or Lumiera’s even more
iter-adapter.hpp provides some helper templates for building Lumiera Forward Iterators.
IterAdapter is the most flexible variant, intended for use by custom facilities. An IterAdapter maintains an internal back-link to a facilitiy exposing an iteration control API, which is accessed through free functions as extension point. This iteration control API is similar to C#, allowing to advance to the next result and to check the current iteration state.
RangeIter wraps two existing iterators — usually obtained from
end()of an STL container embedded within the implementation. This allows for iterator chaining.
PtrDerefIter works similar, but can be used on an STL container holding pointers, to be dereferenced automatically on access
Similar to the STL habits, Lumiera Forward Iterators should expose typedefs for
value_type. Additionally, they may be used for
resource management purposes by “hiding” a ref-counting facility, e.g.
allowing to keep a snapshot or restult set around until it can’t be accessed
The concept was implemented both for unit test and to be used on the QueryResolver facility; thus it can be expected to show up on the session interface, as the PlacementIndex implements QueryResolver. QueryFocus also relies on that interface for discovering session contents. Besides that, we need more implementation experience.
Some existing iterators or collection-style interfaces should be retro-fitted.
See Ticket #349.
Moreover, we need to gain experience about mapping this concept down into a flat C-style API.
expose pointers or arrays
inherit from an Iterator ABC
unfold the iteration control functions into the custom types
define a selection of common container types to be allowed on APIs
use active iteration, i.e. pass a closure or callback
APIs should be written such as not tie them to the current implementation. Exposing iterators is known to create a strong incentive in this direction and thus furthers the creation of clean APIs.
Especially in Proc-Layer we utilise already several iterator implementations, but without an uniform concept, these remain just slightly disguised implementation types of a specific container. Moreover, the STL defines various and very elaborate iterator concepts. Ichthyo considers most of these an overkill and an outdated aproach. Many modern programming languages build with success on a very simple iterator concept, which allows once to pull a sequence of values — and nothing more.
Thus the idea is to formulate a concept in compliance with STL’s forward iterator — but augmented by an stop-iteration test. This would give us basic STL integration and look familiar to C++ and Java programmers without compromising the clean APIs.
Now in use since more then a year, without turning up any serious problems. The only minor concern I can see is that this concept, as such, doesn’t solve the problem with exposing implementation details of the underlying container on the API. Similar to STL Iterators, the actual implementation representation is only disguised behind a typedef. But, generally speaking, this is an inevitable consequence of the “zero overhead” abstraction. For the cases when an indirection (via VTable) is feasible, I’ve created the IterSource template, which sticks to this Lumiera Forward Iterator concept, but provides an opaque frontend, allowing to decouple completely from the actual implementation. Besides that, over time I’ve written several standard adapters for the most common STL containers, plus Map, key and value extractors.
Sa 16 Apr 2011 00:20:13 CEST
minor change: removed support for post-increment. It doesn’t fit with the concept and caused serious problems in practice. A correct implementation of post-increment would require a “deep copy” of any underlying data structures.
Sa 07 Jan 2012 21:49:09 CET <email@example.com>
|Lumiera Forward Iterators can be made to work together with the
range for loop, as introduced with C++11. The preferred solution
is to define the necessary free functions
This is to say, these two concepts can be made to work together well. While, at a conceptual level, they are not really compatible, and build on a different understanding: The standard for-loop assumes “a container”, while our Forward Iterator Concept deals with “abstract data sources”. The user needs to understand the fine points of that conceptual difference:
if you apply the range-
forconstruct on a container, you can iterate as often as you like. Even if the iterators of that container are implemented in compliance with the Lumiera Forward Iterator concept.
but if you apply the range-
forconstruct on a given iterator, you can do so only once. There is no way to reset that iterator, other than obtaining a new one.
71167be9c9aaa for the typical bridge implementation
Sa 04 Jul 2015 22:57:51 CEST <firstname.lastname@example.org>
Accepted on developer meeting
Do 14 Apr 2011 02:52:30 CEST Christian Thaeter
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