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.\" ========================================================================
.\"
.IX Title "Tangram::Complicity 3pm"
.TH Tangram::Complicity 3pm "2015-10-09" "perl v5.20.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
Tangram::Complicity \- How to make Tangram\-friendly classes
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  package YourNastyXSClass;
\&
\&  sub px_freeze {
\&      return [ (shift)\->gimme_as_perl ];
\&  }
\&
\&  sub px_thaw {
\&      my $class = shift;
\&      my $self = $class\->new( @_ );
\&  }
\&
\&  1;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBTangram::Complicity\fR does not exist.  To make matters worse, it
isn't even implemented.  This page is a big \s-1FIXME\s0 for the code it
refers to.  This page merely documents the \s-1API\s0 that classes must
implement to be safely stored by \f(CW\*(C`Tangram::Type::Dump::flatten\*(C'\fR.
.PP
Note that to avoid unnecessary copying of memory structures from A to
B, this method operates \*(L"in-place\*(R".
.PP
So, therefore it is necessary for the reference type used in the
return value, to be the same as the one in the real object.  This is
explained later under \*(L"reftype mismatch\*(R".
.PP
So \- for instance, for Set::Object objects, which have a
\&\f(CW\*(C`px_freeze\*(C'\fR method of:
.PP
.Vb 4
\&  sub px_freeze {
\&      my $self = shift;
\&      return $self\->members;
\&  }
\&
\&  sub px_thaw {
\&      my $class = shift;
\&      return $class\->new(@_);
\&  }
.Ve
.PP
[ note: This differs from the Storable \s-1API \s0(\f(CW\*(C`STORABLE_freeze\*(C'\fR and
\&\f(CW\*(C`STORABLE_thaw\*(C'\fR).  This interface is actually reasonably sane \- the
Storable \s-1API\s0 required custom \s-1XS\s0 magic for Set::Object, for instance.
Which has been implemented, but we've learned the lesson now :) ]
.PP
In essence, the \f(CW\*(C`px_freeze\*(C'\fR method means \*(L"marshall yourself to pure
Perl data types\*(R".  Note that different serialisation tools will treat
ties, overload and magic remaining on the structure in their own way \-
so, create your own type of magic (a la Pixie::Info) if you really
want to hang out-of-band information off them.
.SS "reftype mismatch"
.IX Subsection "reftype mismatch"
If you get a \f(CW\*(C`reftype mismatch\*(C'\fR error, it is because your
\&\fBYourClass\->px_thaw\fR function returned a different type of
reference than the one that was passed to store to
\&\fBYourClass\->px_freeze\fR.
.PP
This restriction only applies to the return value of the constructor
\&\f(CW\*(C`px_thaw\*(C'\fR, so this is usually fine.  The return value from
\&\f(CW\*(C`px_freeze\*(C'\fR will be wrapped in a (blessed) container of the correct
reference type, regardless of its return type.
.PP
ie. your function is called as:
.PP
.Vb 1
\&   %{ $object } = %{ YourClass\->px_thaw(@icicle) };
\&
\&   @{ $object } = @{ YourClass\->px_thaw(@icicle) };
\&
\&   ${ $object } = ${ YourClass\->px_thaw(@icicle) };
\&
\&   *{ $object } = *{ YourClass\->px_thaw(@icicle) };
\&
\&   my $tmp = YourClass\->px_thaw(@icicle);
\&   $object = sub { goto $tmp };
.Ve
.PP
This is an analogy, no temporary object is actually used in the scalar
case, for instance; due to the use of tie.
.PP
The reason for this is to allow for circular and back-references in
the data structure; those references that point back point to the real
blessed object, so to avoid the overhead of a two-pass algorithm, this
restriction is made.  This is why the value is passed into
STORABLE_thaw as \f(CW$_\fR[0].  For most people, it won't make a difference.
.PP
However, it \fIdoes\fR have the nasty side effect that serialisers that
can't handle all types of pure Perl data structures (such as, all
current versions of \s-1YAML\s0) are unable to store blessed scalars (eg,
Set::Object's).