On 3 Apr 2006, at 11:29, Peter Sommerlad wrote:
Hi James, hi all
Siddle, James wrote:
Hi Peter,
Could you recommend an alternative approach to singleton when
applying cross
cutting concerns to legacy code?
Let me elaborate...the C++ project I'm currently working provides
a nice
framework & general approach to accessing the current 'context',
e.g. an
element sitting in the framework can get hold of a variety of
objects via a
"framework context". This is an interface that is actually
provided by the
overall framework container.
However, the project must also provide exception handling &
logging to a
large quantity of legacy code...so we wrote a couple of singleton
classes to
fulfil those roles. Are there any alternatives you could recommend
to this
approach?
in a similar context we used a Context object passed through all
method calls in the framework. This collected all globally needed
information. There is also a pattern called Execution Context by
Alan Kelly if I remember correctly, describing that mechanism.
see http://www.allankelly.net/patterns/#EncapsulateContext
And that's effectively a simulation of what some programming
languages provide in the form of dynamically scoped variables. They
can be understood as global variables, but without (some of) the
disadvantages. For example, you can typically rebind such variables
for the current thread without affecting other threads.
Dynamically scoped variables come in different flavors. In Common
Lisp, they are called special variables - see http://p-cos.net/
documents/dynfun.pdf ; in Scheme fluid variables or parameter objects
- see http://srfi.schemers.org/srfi-39/ ; in Haskell and C++ implicit
parameters - see http://citeseer.ist.psu.edu/lewis00implicit.html and
http://citeseer.ist.psu.edu/hanson01dynamic.html ; Perl has them, but
I don't recall the details; in Java, they can be simulated with
thread-local variables; etc.
Pascal
--
Pascal Costanza, mailto:[EMAIL PROTECTED], http://p-cos.net
Vrije Universiteit Brussel, Programming Technology Lab
Pleinlaan 2, B-1050 Brussel, Belgium
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