Provided by: tcllib_1.19-dfsg-2_all 
NAME
switched - switch/option management.
SYNOPSIS
package require Tcl 8.3
package require switched ?2.2.1?
<switched> complete this
<switched> options this
<switched> set-option this value
________________________________________________________________________________________________________________
DESCRIPTION
The switched class serves as base class for user classes with switch / option configuration procedures.
It provides facilities for managing options through a simple interface.
For example:
set vehicle [new car -length 4.5 -width 2 -power 100 -fuel diesel]
puts "my car was running on [switched::cget $vehicle -fuel]"
switched::configure $vehicle -power 40 -fuel electricity
puts "but is now running on clean [switched::cget $vehicle -fuel]"
Of course, as you might have guessed, the car class is derived from the switched class. Let us see how it
works:
class car {
proc car {this args} switched {$args} {
# car specific initialization code here
switched::complete $this
}
...
}
The switched class constructor takes the optional configuration option / value pairs as parameters. The
switched class layer then completely manages the switched options: it checks their validity, stores their
values and provides a clean interface to the user layer configuration setting procedures.
The switched class members available to the programmer are:
<switched> complete this
This procedure is used to tell the switched layer that the derived class object (a car in the
examples) is completely built. At that time, the initial configuration of the switched object
occurs, using default option values (see procedure options) eventually overridden by construction
time values, passed at the time of the new operator invocation. This procedure must be called
only once, usually around or at the end of the derived class constructor. (Note: Also check the
complete data member later in this chapter).
<switched> options this
This procedure must return the configuration description for all options that the switched object
will accept. It is a pure virtual member procedure and therefore its implementation is mandatory
in the derived class layer. The procedure must return a list of lists. Each list pertains to a
single option and is composed of the switch name, the default value for the option and an optional
initial value. For example:
class car {
...
proc options {this} {
return [list [list -fuel petrol petrol] [list -length {} {}] [list -power {} {}] [list -width {} {}] ]
}
proc set-fuel {this value} {
...
}
...
}
In this case, 4 options are specified: fuel, length, power and width. The default and initial values for
the fuel option are identical and set to petrol. For the other options, values are all empty.
For each option, there must be a corresponding set-option procedure defined in the derived class layer.
For example, since we defined a fuel option, there is a set-fuel procedure in the car class. The
parameters always are the object identifier (since this is not a static procedure, but rather a
dynamically defined virtual one), followed by the new value for the option. A set-option procedure is
only invoked if the new value differs from the current one (a caching scheme for improving performance),
or if there is no initial value set in the options procedure for that option.
In this procedure, if the initial value differs from the default value or is omitted, then initial
configuration is forced and the corresponding set-option procedure is invoked by the switched complete
procedure located at the end of the derived class constructor. For example:
class car {
...
proc options {this} {
return [list [list -fuel petrol] [list -length {} {}] [list -power 100 50] [list -width {} {}] ]
}
...
}
In this case, configuration is forced on the fuel and power options, that is the corresponding
set-option procedures will be invoked when the switched object is constructed (see set-option
procedures documentation below).
For the fuel option, since there is no initial value, the set-fuel procedure is called with the
default value (petrol) as argument. For the power option, since the initial value differs from
the default value, the set-power procedure is called with the initial value as argument (50).
For the other options, since the initial values (last elements of the option lists) are identical
to their default values, the corresponding set-option procedures will not be invoked. It is the
programmer's responsibility to insure that the initial option values are correct.
<switched> set-option this value
These procedures may be viewed as dynamic virtual functions. There must be one implementation per
supported option, as returned by the options procedure. For example:
class car {
...
proc options {this} {
return [list ...
[list -width {} {}] ]
}
...
proc set-width {this value} {
...
}
...
}
Since the -width option was listed in the options procedure, a set-width procedure implementation
is provided, which of course would proceed to set the width of the car (and would modify the looks
of a graphical representation, for example).
As you add a supported option in the list returned by the options procedure, the corresponding
set-option procedure may be called as soon as the switched object is complete, which occurs when
the switched level complete procedure is invoked. For example:
class car {
proc car {this args} switched {args} {
...
switched::complete $this
}
...
proc options {this} {
return [list [list -fuel petrol] [list -length 4.5] [list -power 350] [list -width 1.8] ]
}
proc set-fuel {this value} {
...
}
proc set-length {this value} {
...
}
proc set-power {this value} {
...
}
proc set-width {this value} {
...
}
}
new car
In this case, a new car is created with no options, which causes the car constructor to be called, which
in turns calls the switched level complete procedure after the car object layer is completely
initialized. At this point, since there are no initial values in any option list in the options
procedure, the set-fuel procedure is called with its default value of petrol as parameter, followed by
the set-length call with 4.5 value, set-power with 350 value and finally with set-width with 1.8 as
parameter. This is a good way to test the set-option procedures when debugging, and when done, just
fill-in the initial option values.
The switched layer checks that an option is valid (that is, listed in the options procedure) but
obviously does not check the validity of the value passed to the set-option procedure, which should throw
an error (for example by using the Tcl error command) if the value is invalid.
The switched layer also keeps track of the options current values, so that a set-option procedure is
called only when the corresponding option value passed as parameter is different from the current value
(see data members description).
The data member is an options current value.
There is one for each option listed in the options procedure. It is a read-only value which the
switched layer checks against when an option is changed. It is rarely used at the layer derived
from switched, except in the few cases, such as in the following example:
...
proc car::options {this} {
return {
...
{-manufacturer {} {}}
...
}
}
proc car::set-manufacturer {this value} {}
proc car::printData {this} {
puts "manufacturer: $switched::($this,-manufacturer)"
...
}
In this case, the manufacturer's name is stored at the switched layer level (this is why the set-
manufacturer procedure has nothing to do) and later retrieved in the printData procedure.
The data member (not to be confused with
the complete procedure) is a boolean. Its initial value is false and it is set to true at the
very end of the switched complete procedure. It becomes useful when some options should be set at
construction time only and not dynamically, as the following example shows:
proc car::set-width {this value} {
if {$switched::($this,complete)} {
error {option -width cannot be set dynamically}
}
...
}
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please
report such in the category stooop of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
also report any ideas for enhancements you may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by
going to the Edit form of the ticket immediately after its creation, and then using the left-most button
in the secondary navigation bar.
KEYWORDS
C++, class, object, object oriented
CATEGORY
Programming tools