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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: ANNOUNCE: command pattern posted to ACM archives
Date: 1999/03/14
Message-ID: <m3zp5gewzl.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
NNTP-Posting-Date: Sat, 13 Mar 1999 16:24:12 PDT
Newsgroups: comp.lang.ada
Date: 1999-03-14T00:00:00+00:00
List-Id: <comp.lang.ada>

I have prepared a short article on how to implement the command pattern
in Ada95, and posted it to the ACM patterns archive.  The introduction
of the article appears below.

<http://www.acm.org/archives/patterns.html>

I've been slowly converting the C++ examples in the book Design Patterns
to Ada95.  In each article I discuss the pattern, explain how to
implement it in Ada, and explore various idioms and language features.
A complete, working example with all the code is included.

You can subscribe to the patterns list by sending a message with the
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subscribe patterns <your full name>

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Matt

Command Pattern

Command objects manage the processing that occurs when a user
manipulates the application in some way.

Objectifying a command admits all kinds of interesting possibilities.
You can put the command on a stack to implement undo and redo, or write
the command to disk to implement record/playback.

You don't even have to execute the command right away.  In a simulation
each command has a scenario time.  You put the command on a queue and
execute the command later, when the simulation reaches that point in the
scenario.

In the example here, a menu-based application manages a group of open
documents.  Each menu item contains a command object, bound at creation
time to a document object, which when executed calls a document
operation.


Implementation

In C++, you can bind a command to a document by passing the document as
a parameter in the constructor for the command.

For binding one object to another in Ada95 we use access discriminants,
a language feature created specifically for this purpose.  The advantage
of this approach over C++ constructors is that the language guarantees
that a dangling reference cannot occur.

<rest of article and code snipped>