'\"
'\" Copyright (c) 2018 Donal K. Fellows
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.TH singleton 3tcl 0.3 TclOO "TclOO Commands"
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'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
oo::singleton \- a class that does only allows one instance of itself
.SH SYNOPSIS
.nf
package require tcl::oo

\fBoo::singleton\fI method \fR?\fIarg ...\fR?
.fi
.SH "CLASS HIERARCHY"
.nf
\fBoo::object\fR
   \(-> \fBoo::class\fR
       \(-> \fBoo::singleton\fR
.fi
.BE
.SH DESCRIPTION
Singleton classes are classes that only permit at most one instance of
themselves to exist. They unexport the \fBcreate\fR and
\fBcreateWithNamespace\fR methods entirely, and override the \fBnew\fR method
so that it only makes a new instance if there is no existing instance.  It is
not recommended to inherit from a singleton class; singleton-ness is \fInot\fR
inherited. It is not recommended that a singleton class's constructor take any
arguments.
.PP
Instances have their\fB destroy\fR method overridden with a method that always
returns an error in order to discourage destruction of the object, but
destruction remains possible if strictly necessary (e.g., by destroying the
class or using \fBrename\fR to delete it). They also have a (non-exported)
\fB<cloned>\fR method defined on them that similarly always returns errors to
make attempts to use the singleton instance with \fBoo::copy\fR fail.
.SS CONSTRUCTOR
The \fBoo::singleton\fR class does not define an explicit constructor; this
means that it is effectively the same as the constructor of the
\fBoo::class\fR class.
.SS DESTRUCTOR
The \fBoo::singleton\fR class does not define an explicit destructor;
destroying an instance of it is just like destroying an ordinary class (and
will destroy the singleton object).
.SS "EXPORTED METHODS"
.TP
\fIcls \fBnew \fR?\fIarg ...\fR?
.
This returns the current instance of the singleton class, if one exists, and
creates a new instance only if there is no existing instance. The additional
arguments, \fIarg ...\fR, are only used if a new instance is actually
manufactured; that construction is via the \fBoo::class\fR class's \fBnew\fR
method.
.RS
.PP
This is an override of the behaviour of a superclass's method with an
identical call signature to the superclass's implementation.
.RE
.SS "NON-EXPORTED METHODS"
The \fBoo::singleton\fR class explicitly states that \fBcreate\fR and
\fBcreateWithNamespace\fR are unexported; callers should not assume that they
have control over either the name or the namespace name of the singleton instance.
.SH EXAMPLE
.PP
This example demonstrates that there is only one instance even though the
\fBnew\fR method is called three times.
.PP
.CS
\fBoo::singleton\fR create Highlander {
    method say {} {
        puts "there can be only one"
    }
}

set h1 [Highlander new]
set h2 [Highlander new]
if {$h1 eq $h2} {
    puts "equal objects"    \fI\(-> prints "equal objects"\fR
}
set h3 [Highlander new]
if {$h1 eq $h3} {
    puts "equal objects"    \fI\(-> prints "equal objects"\fR
}
.CE
.PP
Note that the name of the instance of the singleton is not guaranteed to be
anything in particular.
.SH "SEE ALSO"
oo::class(3tcl)
.SH KEYWORDS
class, metaclass, object, single instance
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