Check-in [8b7522e1b5]

A supporting introspector is also added, **info object creationid**, which
returns the creation ID of any existing object. It also applies to classes.
Again, note that creation IDs are _always_ system-allocated and are _never_
guaranteed to be unique between interpreters, either in multiple processes or
in the same thread or process; they are only ever locally unique.

# Examples



This example shows a private variable linked to a Tk entry:

    oo::class create Editable {
        variable w;            # Allow subclasses to see this
        private variable val;  # Hide this from subclasses

................................................................................
    }

    Editable create field .e
    field trace {args {
        puts "field is now [field value]"
    }}
    field value "Set the contents"



Ths is an example of private methods. It shows how private methods can be used
to manage the complexity of a method without making its API (as exposed to
either subclasses or the rest of Tcl) overly complex.

    oo::class create Modulator {
        # The exported interface to the class
................................................................................
    #        invoked from within
    #    "::oo::Obj15 modulate 5 6 7 8"

    catch {$m no.such.method} msg
    puts $msg
    # Prints something like:
    #    unknown method "no.such.method": must be destroy or modulate



This is a combined example of private methods and variables.

    oo::class create LabelEqual {
        constructor {label} {
            set [my varname label] $label
        }
................................................................................
    puts "one=[$expr1 value] two=[$expr2 value]"
    # Prints:
	#    one=6 two=6

    puts [info vars [info object namespace $expr1]::*]
    # Prints something like:
	#    {::oo::Obj13::11 : label} ::oo::Obj13::label



This example highlights the behaviour of private variables and
`info object creationid`:

    # A simple introspection procedure for classes
    proc dumpinfo cls {
        puts "class ID of $cls is [info object creationid $cls]"

A supporting introspector is also added, **info object creationid**, which
returns the creation ID of any existing object. It also applies to classes.
Again, note that creation IDs are _always_ system-allocated and are _never_
guaranteed to be unique between interpreters, either in multiple processes or
in the same thread or process; they are only ever locally unique.

# Examples

## Example: Private Variables and Tk

This example shows a private variable linked to a Tk entry:

    oo::class create Editable {
        variable w;            # Allow subclasses to see this
        private variable val;  # Hide this from subclasses

................................................................................
    }

    Editable create field .e
    field trace {args {
        puts "field is now [field value]"
    }}
    field value "Set the contents"

## Example: Private Methods and Code Complexity Management

Ths is an example of private methods. It shows how private methods can be used
to manage the complexity of a method without making its API (as exposed to
either subclasses or the rest of Tcl) overly complex.

    oo::class create Modulator {
        # The exported interface to the class
................................................................................
    #        invoked from within
    #    "::oo::Obj15 modulate 5 6 7 8"

    catch {$m no.such.method} msg
    puts $msg
    # Prints something like:
    #    unknown method "no.such.method": must be destroy or modulate

## Example: Private Methods and Variables Working Together

This is a combined example of private methods and variables.

    oo::class create LabelEqual {
        constructor {label} {
            set [my varname label] $label
        }
................................................................................
    puts "one=[$expr1 value] two=[$expr2 value]"
    # Prints:
	#    one=6 two=6

    puts [info vars [info object namespace $expr1]::*]
    # Prints something like:
	#    {::oo::Obj13::11 : label} ::oo::Obj13::label

## Example: Private Variable Naming and Creation IDs

This example highlights the behaviour of private variables and
`info object creationid`:

    # A simple introspection procedure for classes
    proc dumpinfo cls {
        puts "class ID of $cls is [info object creationid $cls]"