The switched class serves as base class for user classes with switch / option configuration method. 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]"

class car {
    proc car {this args} switched {$args} {
        # car specific initialization code here
        switched::complete $this
    }
    ...
}

The switched class members available to the programmer are:

switched

• complete{}
• options{}
• set-option{}
• ...
• complete
• -option
• ...

The options 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} {
        ...
    }
    ...
}

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. The set-option procedure is only invoked if the new value differs from the current one (a cache scheme for improving performance), or if there is no initial value set in the options procedure for that option.

In the options 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 {} {}]\
        ]
    }
    ...
}

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.

The set-option 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} {
        ...
    }
    ...
}

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

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 -option data members description).

The -option data member is the option 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)"
    ...
}

The complete 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}
    }
    ...
}