Re: Pattern for "make"-like dependencies between objects

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 25 Nov 2007 09:32:18 -0800 (PST)
Message-ID:
<90d3362b-07ba-46a2-aa20-e9826b00e3de@r31g2000hsg.googlegroups.com>
On Nov 24, 11:26 pm, Phil Endecott
<spam_from_usenet_0...@chezphil.org> wrote:

alan wrote:

On Nov 24, 11:15 pm, Phil Endecott
<spam_from_usenet_0...@chezphil.org> wrote:

Say I have an application with various data inputs (files,
network connections, peripherals etc), various data outputs
(files, display etc.) and various intermediate internal
data objects. I would like changes in the inputs to
propogate through to the outputs. In my mind, I'm
imagining the dependencies between the objects like "make"
dependencies.

There are various ways of doing this, including "push" and
"pull" methods, with various pros and cons. Is anyone
aware of any literature ("patterns") describing something
like this?

One thought that has occured to me is that C++ has built-in
compile-time dependency graph logic in the multiple
inheritance constructors. So, say I have these objects:

DataSource src;
InternalObj obj1;
InternalObj obj2;
Display disp;

If obj1 and obj2 get their data from the DataSrc, and the
Display gets its data from obj1 and obj2 (i.e. a
diamond-shape dependency graph), and each object has an
"update" method, then I can write:

struct make_src {
   make_src() { src.update(); }
};

struct make_obj1: make_src {
   make_obj1() { obj1.update(); }
};

struct make_obj2: make_src {
   make_obj2() { obj2.update(); }
};

struct make_disp: virtual make_obj1, virtual make_obj2 {
   make_disp { disp.update(); }
};

Now, when I want to update the display I can just create a
temporary make_disp object, and all of the update() methods
will be called in the right order. (I think. I'm a bit
rusty on what the virtual base classes do. I hope that
src.update() gets called only once.)


Correct.

I bet I'm not the first one to think of this, but google
doesn't find anything - perhaps because of the zillions of
hits for extracting #include dependencies for make from C++
source.

Any thoughts? What else is needed to make this useful?
For example, what's the best way to tag objects with the
equivalent of make's timestamps?


Use a cache with a "dirty" bit to indicate that the cache's
contents are no longer valid. Each object's update() method
then checks if the cache is clean, and if so returns the
data in the cache. Otherwise it calls the data sources'
update() functions and performs the computation for that
node, then stores the result in the cache and cleans the
cache.


I don't think that single bits are sufficient in general, e.g.
if B depends on A and C also depends on A, and A changes, and
I ask to re-make B but not C, what happens? I need to record
that C is out-of-date wrt A but B is up-to-date wrt A.

Single timestamps (like make) also don't work in the case
where C is generated from B and B is generated from A, but
some changes to A don't influence B (e.g. B is a sub-set of
A).


That's the classical problem of the granularity of make. Update
a comment in a header, and watch the entire system get
rebuilt:-). You can use a variant of the classical solution
used in makefiles: do a diff on the object with its old value
before updating its timestamp. Thus, B knows whether the
changes to A have modified it or not; it only updates its last
modified timestamp if they have. (In makefiles, this is
classically used when a program generates both a header and a
source file, and the header changes very rarely. The rules in
make generate the header locally, then do something like:
    cmp local.h target.h || cp local.h target.h
. Other rules only look at the timestamp for target.h.)

In this case, when I re-make C, I first re-make B from A; but
if B doesn't change I then don't need to update C.


Nor update the timestamp associated with B.

So I need to record that B is update-to-date with the latest
version of A, but has not changed since some previous time.
Can this be solved with just a pair of timestamps, one for
"last changed" and one for "last updated"?


Do you even need the last updated?

    [...]

One interesting feature of the structs that I describe above
is that they're non-intrusive of the objects whose updates
they control, which seems like a nice feature. It's also
entirely compile-time.

I'd love to hear some feedback about that idea, by the way.


It's interesting, since it involves letting the compiler manage
the DAG, rather than doing it manually at run-time. On the
other hand, I'm not too sure about its readability. It looks a
little too subtle to me. You'd need a lot of comments, at any
rate.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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