For context: I've engaged into writing a `LazyInit` helper component,
to resolve the inner contradiction between DSL use of `RandomDraw`
(implying value semantics) and the design of a processing pipeline,
which quite naturally leads to binding by reference into the enclosing
implementation.
In most cases, this change (to lazy on-demand initialisation) should be
transparent for the complete implementation code in `RandomDraw` -- with
one notable exception: when configuring an elaborate pipeline, especially
with dynamic changes of the probability profile during the simulation run,
then then obviously there is the desire to use the existing processing
pipeline from the reconfiguration function (in fact it would be quite
hard to explain why and where this should be avoided). `LazyInit` breaks
this usage scenario, since -- at the time the reconfiguration runs --
now the object is not initialised at all, but holds a »Trojan« functor,
which will trigger initialisation eventually.
After some headaches and grievances (why am I engaging into such an
elaborate solution for such an accidental and marginal topic...),
unfortunately it occurred to me that even this problem can be fixed,
with yet some further "minimal" adjustments to the scheme: the LazyInit
mechanism ''just needs to ensure'' that the init-functor ''sees the
same environment as in eager init'' -- that is, it must clear out the
»Trojan« first, and it ''could apply any previous pending init function''
fist. That is, with just a minimal change, we possibly build a chain
of init functors now, and apply them in given order, so each one
sees the state the previous one created -- as if this was just
direct eager object manipulation...
...this is a more realistic demo example, which mimics
some of the patterns present in RandomDraw. The test also
uses lambdas linking to the actual storage location, so that
the invocation would crash on a copy; LazyInit was invented
to safeguard against this, while still allowing leeway
during the initialisation phase in a DSL.
...oh my.
This is getting messy. I am way into danger territory now....
I've made a nifty cool design with automatically adapted functors;
yet at the end of the day, this does not bode well with a DSL usage,
where objects appear to be simple values from a users point of view.
- Helper function to find out of two objects are located
"close to each other" -- which can be used as heuristics
to distinguish heap vs. stack storage
- further investigation shows that libstdc++ applies the
small-object optimisation for functor up to »two slots«
in size -- but only if the copy-ctor is trivial. Thus
a lambda capturing a shared_ptr by value will *always*
be maintained in heap storage (and LazyInit must be
redesigned accordingly)...
- the verify_inlineStorage() unit test will now trigger
if some implementation does not apply small-object optimisation
under these minimal assumptions
the RandomDraw rules developed last days are meant to be used
with user-provided λ-adapters; employing these in a context
of a DSL runs danger of producing dangling references.
Attempting to resolve this fundamental problem through
late-initialisation, and then locking the component into
a fixed memory location prior to actual usage. Driven by
the goal of a self-contained component, some advanced
trickery is required -- which again indicates better
to write a library component with adequate test coverage.
