Re: [PATCH] Hash::Util::FieldHash

Message-Id: <974A5B4B-7614-4F3F-BA7C-828960D82C55@mailbox.tu-berlin.de>

p4raw-id: //depot/perl@28419

15 files changed, 1992 insertions, 2 deletions

diff --git a/ext/Hash/Util/Changes b/ext/Hash/Util/Changes
index f6ba16bf4a..06589b56ff 100644
--- a/ext/Hash/Util/Changes
+++ b/ext/Hash/Util/Changes
@@ -15,4 +15,6 @@ on top of code by Nick Ing-Simmons and Jeffrey Friedl.
 
 
 
-
+0.07 Sun Jun 11 21:24:15 CEST 2006
+        - added front-end support for the new Hash::Util::FieldHash
+        (Anno Siegel)
diff --git a/ext/Hash/Util/FieldHash/Changes b/ext/Hash/Util/FieldHash/Changes
new file mode 100644
index 0000000000..071dcaa4ff
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/Changes
@@ -0,0 +1,6 @@
+Revision history for Perl extension Hash::Util::FieldHash.
+
+0.01  Sat Jun  3 16:24:12 2006
+	- original version; created by h2xs 1.23 with options
+		-A -g --skip-ppport -nHash::Util::FieldHash
+
diff --git a/ext/Hash/Util/FieldHash/FieldHash.xs b/ext/Hash/Util/FieldHash/FieldHash.xs
new file mode 100644
index 0000000000..32c936180b
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/FieldHash.xs
@@ -0,0 +1,353 @@
+#include "EXTERN.h"
+#include "perl.h"
+#include "XSUB.h"
+
+/* support for Hash::Util::FieldHash, prefix HUF_ */
+
+/* The object registry, a package variable */
+#define HUF_OB_REG "Hash::Util::FieldHash::ob_reg"
+/* Magic cookies to recognize object id's.  Hi, Eva, David */
+#define HUF_COOKIE 2805.1980
+#define HUF_REFADDR_COOKIE 1811.1976
+
+
+/* For global cache of object registry */
+#define MY_CXT_KEY "Hash::Util::FieldHash::_guts" XS_VERSION
+typedef struct {
+    HV* ob_reg; /* Cache object registry */
+} my_cxt_t;
+START_MY_CXT
+
+/* Deal with global context */
+#define HUF_INIT 1
+#define HUF_CLONE 0
+#define HUF_RESET -1
+
+void HUF_global(I32 how) {
+    if (how == HUF_INIT) {
+        MY_CXT_INIT;
+        MY_CXT.ob_reg = get_hv(HUF_OB_REG, 1);
+    } else if (how == HUF_CLONE) {
+        MY_CXT_CLONE;
+        MY_CXT.ob_reg = get_hv(HUF_OB_REG, 0);
+    } else if (how == HUF_RESET) {
+        dMY_CXT;
+        MY_CXT.ob_reg = get_hv(HUF_OB_REG, 0);
+    }
+}
+
+/* the id as an SV, optionally marked in the nv (unused feature) */
+SV* HUF_id(SV* ref, NV cookie) {
+    SV* id = sv_newmortal();
+    if (cookie == 0 ) {
+        SvUPGRADE(id, SVt_PVIV);
+    } else {
+        SvUPGRADE(id, SVt_PVNV);
+        SvNV_set(id, cookie);
+        SvNOK_on(id);
+    }
+    SvIV_set(id, (IV)SvRV(ref));
+    SvIOK_on(id);
+    return id;
+}
+
+/* plain id, only used for field hash entries in field lists */
+SV* HUF_field_id(SV* obj) {
+    return HUF_id(obj, 0.0);
+}
+
+/* object id (may be different in future) */
+SV* HUF_obj_id(SV* obj) {
+    return HUF_id(obj, 0.0);
+}
+
+/* set up uvar magic for any sv */
+void HUF_add_uvar_magic(
+    SV* sv,                    /* the sv to enchant, visible to * get/set */
+    I32(* val)(pTHX_ IV, SV*), /* "get" function */
+    I32(* set)(pTHX_ IV, SV*), /* "set" function */
+    I32 index,                 /* get/set will see this */
+    SV* thing                  /* any associated info */
+) {
+    MAGIC* mg;
+    struct ufuncs uf;
+        uf.uf_val = val;
+        uf.uf_set = set;
+        uf.uf_index = index;
+    sv_magic(sv, thing, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
+}
+
+/* Fetch the data container of a trigger */
+AV* HUF_get_trigger_content(SV* trigger) {
+    MAGIC* mg;
+    if (trigger && (mg = mg_find(trigger, PERL_MAGIC_uvar)))
+        return (AV*)mg->mg_obj;
+    return NULL;
+}
+
+/* Delete an object from all field hashes it may occur in.  Also delete
+ * the object's entry from the object registry.
+ */
+I32 HUF_destroy_obj(pTHX_ IV index, SV* trigger) {
+    /* Do nothing if the weakref wasn't undef'd.  Also don't bother
+     * during global destruction.  (MY_CXT.ob_reg is sometimes funny there) */
+    if (!SvROK(trigger) && (!PL_in_clean_all)) {
+        dMY_CXT;
+        AV* cont = HUF_get_trigger_content(trigger);
+        SV* ob_id = *av_fetch(cont, 0, 0);
+        HV* field_tab = (HV*) *av_fetch(cont, 1, 0);
+        HE* ent;
+        hv_iterinit(field_tab);
+        while (ent = hv_iternext(field_tab)) {
+            SV* field_ref = HeVAL(ent);
+            SV* field = SvRV(field_ref);
+            hv_delete_ent((HV*)field, ob_id, G_DISCARD, 0);
+        }
+        /* make it safe in case we must run in global clenaup, after all */
+        if (PL_in_clean_all)
+            HUF_global(HUF_RESET);
+        hv_delete_ent(MY_CXT.ob_reg, ob_id, G_DISCARD, 0);
+    }
+    return 0;
+}
+
+/* Create a trigger for an object.  The trigger is a magical weak ref
+ * that fires when the weak ref expires.  it holds the original id of
+ * the object, and a list of field hashes from which the object may
+ * have to be deleted.  The trigger is stored in the object registry
+ * and also deleted when the object expires.
+ */
+SV* HUF_new_trigger(SV* obj, SV* ob_id) {
+    dMY_CXT;
+    SV* trigger = sv_rvweaken(newRV_inc(SvRV(obj)));
+    AV* cont = newAV();
+    sv_2mortal((SV*)cont);
+    av_store(cont, 0, SvREFCNT_inc(ob_id));
+    av_store(cont, 1, (SV*)newHV());
+    HUF_add_uvar_magic(trigger, NULL, &HUF_destroy_obj, 0, (SV*)cont);
+    hv_store_ent(MY_CXT.ob_reg, ob_id, trigger, 0);
+    return trigger;
+}
+
+/* retrieve a trigger for obj if one exists, return NULL otherwise */
+SV* HUF_ask_trigger(SV* ob_id) {
+    dMY_CXT;
+    HE* ent;
+    if (ent = hv_fetch_ent(MY_CXT.ob_reg, ob_id, 0, 0))
+        return HeVAL(ent);
+    return NULL;
+}
+
+/* get the trigger for an object, creating it if necessary */
+SV* HUF_get_trigger(SV* obj, SV* ob_id) {
+    SV* trigger;
+    if (!(trigger = HUF_ask_trigger(ob_id)))
+        trigger = HUF_new_trigger(obj, ob_id);
+    return trigger;
+}
+
+/* mark an object (trigger) as having been used with a field */
+void HUF_mark_field(SV* trigger, SV* field) {
+    AV* cont = HUF_get_trigger_content(trigger);
+    HV* field_tab = (HV*) *av_fetch(cont, 1, 0);
+    SV* field_ref = newRV_inc(field);
+    SV* field_id = HUF_field_id(field_ref);
+    hv_store_ent(field_tab, field_id, field_ref, 0);
+}
+
+/* The key exchange function.  It communicates with S_hv_magic_uvar_xkey
+ * in hv.c */
+IV HUF_watch_key(pTHX_ IV action, SV* field) {
+    MAGIC* mg = mg_find(field, PERL_MAGIC_uvar);
+    SV* keysv = mg->mg_obj;
+    if (keysv && SvROK(keysv)) {
+        SV* ob_id = HUF_obj_id(keysv);
+        SV* trigger = HUF_get_trigger(keysv, ob_id);
+        HUF_mark_field(trigger, field);
+        mg->mg_obj = ob_id; /* key replacement */
+    }
+    return 0;
+}
+
+/* see if something is a field hash */
+int HUF_get_status(HV* hash) {
+    int ans = 0;
+    if (hash && (SvTYPE(hash) == SVt_PVHV)) {
+        dMY_CXT;
+        MAGIC* mg;
+        struct ufuncs* uf;
+        ans = (mg = mg_find((SV*)hash, PERL_MAGIC_uvar)) &&
+            (uf = (struct ufuncs *)mg->mg_ptr) &&
+            (uf->uf_val == &HUF_watch_key) &&
+            (uf->uf_set == NULL);
+    }
+    return ans;
+}
+
+/* Thread support.  These routines are called by CLONE (and nothing else) */
+
+/* Fix entries for one object in all field hashes */
+void HUF_fix_trigger(SV* trigger, SV* new_id) {
+    AV* cont = HUF_get_trigger_content(trigger);
+    HV* field_tab = (HV*) *av_fetch(cont, 1, 0);
+    HV* new_tab = newHV();
+    HE* ent;
+    SV* old_id = *av_fetch(cont, 0, 0);
+    hv_iterinit(field_tab);
+    while (ent = hv_iternext(field_tab)) {
+        SV* field_ref = HeVAL(ent);
+        SV* field_id = HUF_field_id(field_ref);
+        HV* field = (HV*)SvRV(field_ref);
+        SV* val;
+        /* recreate field tab entry */
+        hv_store_ent(new_tab, field_id, SvREFCNT_inc(field_ref), 0);
+        /* recreate field entry, if any */
+        if (val = hv_delete_ent(field, old_id, 0, 0))
+            hv_store_ent(field, new_id, SvREFCNT_inc(val), 0);
+    }
+    /* update the trigger */
+    av_store(cont, 0, SvREFCNT_inc(new_id));
+    av_store(cont, 1, (SV*)new_tab);
+}
+
+/* Go over object registry and fix all objects.  Also fix the object
+ * registry.
+ */
+void HUF_fix_objects() {
+    dMY_CXT;
+    I32 i, len;
+    HE* ent;
+    AV* oblist = (AV*)sv_2mortal((SV*)newAV());
+    hv_iterinit(MY_CXT.ob_reg);
+    while(ent = hv_iternext(MY_CXT.ob_reg))
+        av_push(oblist, SvREFCNT_inc(hv_iterkeysv(ent)));
+    len = av_len(oblist);
+    for (i = 0; i <= len; ++i) {
+        SV* old_id = *av_fetch(oblist, i, 0);
+        SV* trigger = hv_delete_ent(MY_CXT.ob_reg, old_id, 0, 0);
+        SV* new_id = HUF_obj_id(trigger);
+        HUF_fix_trigger(trigger, new_id);
+        hv_store_ent(MY_CXT.ob_reg, new_id, SvREFCNT_inc(trigger), 0);
+    }
+}
+
+/* test support (not needed for functionality) */
+
+static SV* counter;
+IV HUF_inc_var(pTHX_ IV index, SV* which) {
+    sv_setiv(counter, 1 + SvIV(counter));
+    return 0;
+}
+
+MODULE = Hash::Util::FieldHash          PACKAGE = Hash::Util::FieldHash
+
+BOOT:
+{
+    HUF_global(HUF_INIT); /* create variables */
+}
+
+int
+_fieldhash(SV* href, int mode)
+PROTOTYPE: $$
+CODE:
+    HV* field;
+    RETVAL = 0;
+    if (mode &&
+        href && SvROK(href) &&
+        (field = (HV*)SvRV(href)) &&
+        SvTYPE(field) == SVt_PVHV
+    ) {
+        HUF_add_uvar_magic(
+            SvRV(href),
+            &HUF_watch_key,
+            NULL,
+            0,
+            NULL
+        );
+        RETVAL = HUF_get_status(field);
+    }
+OUTPUT:
+    RETVAL
+
+void
+CLONE(char* class)
+CODE:
+    if (0 == strcmp(class, "Hash::Util::FieldHash")) {
+        HUF_global(HUF_CLONE);
+        HUF_fix_objects();
+    }
+
+SV*
+_get_obj_id(SV* obj)
+CODE:
+    RETVAL = NULL;
+    if (SvROK(obj))
+        RETVAL = HUF_obj_id(obj);
+OUTPUT:
+    RETVAL
+
+SV*
+_active_fields(SV* obj)
+PPCODE:
+    if (SvROK(obj)) {
+        SV* ob_id = HUF_obj_id(obj);
+        SV* trigger = HUF_ask_trigger(ob_id);
+        if (trigger) {
+            AV* cont = HUF_get_trigger_content(trigger);
+            HV* field_tab = (HV*) *av_fetch(cont, 1, 0);
+            HE* ent;
+            hv_iterinit(field_tab);
+            while (ent = hv_iternext(field_tab)) {
+                HV* field = (HV*)SvRV(HeVAL(ent));
+                if (hv_exists_ent(field, ob_id, 0))
+                    XPUSHs(sv_2mortal(newRV_inc((SV*)field)));
+            }
+        }
+    }
+
+void
+_test_uvar_get(SV* svref, SV* countref)
+CODE:
+    if (SvROK(svref) && SvROK(countref)) {
+        counter = SvRV(countref);
+        sv_setiv(counter, 0);
+        HUF_add_uvar_magic(
+            SvRV(svref),
+            &HUF_inc_var,
+            NULL,
+            0,
+            SvRV(countref)
+        );
+    }
+
+void
+_test_uvar_set(SV* svref, SV* countref)
+CODE:
+    if (SvROK(svref) && SvROK(countref)) {
+        counter = SvRV(countref);
+        sv_setiv(counter, 0);
+        counter = SvRV(countref);
+        HUF_add_uvar_magic(
+            SvRV(svref),
+            NULL,
+            &HUF_inc_var,
+            0,
+            SvRV(countref)
+        );
+    }
+
+void
+_test_uvar_same(SV* svref, SV* countref)
+CODE:
+    if (SvROK(svref) && SvROK(countref)) {
+        counter = SvRV(countref);
+        sv_setiv(counter, 0);
+        HUF_add_uvar_magic(
+            SvRV(svref),
+            &HUF_inc_var,
+            &HUF_inc_var,
+            0,
+            NULL
+        );
+    }
+
diff --git a/ext/Hash/Util/FieldHash/Makefile.PL b/ext/Hash/Util/FieldHash/Makefile.PL
new file mode 100644
index 0000000000..1ec06efe27
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/Makefile.PL
@@ -0,0 +1,20 @@
+use 5.009004;
+use ExtUtils::MakeMaker;
+# See lib/ExtUtils/MakeMaker.pm for details of how to influence
+# the contents of the Makefile that is written.
+WriteMakefile(
+    NAME              => 'Hash::Util::FieldHash',
+    VERSION_FROM      => 'lib/Hash/Util/FieldHash.pm', # finds $VERSION
+    PREREQ_PM         => {}, # e.g., Module::Name => 1.1
+    (grep( /^PERL_CORE=1$/, @ARGV) ?  (MAN3PODS => {}) : ()),
+    ($] >= 5.005 ?     ## Add these new keywords supported since 5.005
+      (ABSTRACT_FROM  => 'lib/Hash/Util/FieldHash.pm', # retrieve abstract from module
+       AUTHOR         => 'Anno Siegel <anno@zrz.tu-berlin.de>') : ()),
+    LIBS              => [''], # e.g., '-lm'
+    DEFINE            => '', # e.g., '-DHAVE_SOMETHING'
+	# Insert -I. if you add *.h files later:
+    INC               => '', # e.g., '-I/usr/include/other'
+	# Un-comment this if you add C files to link with later:
+    # OBJECT            => '$(O_FILES)', # link all the C files too
+    CCFLAGS             => '-Wuninitialized',
+);
diff --git a/ext/Hash/Util/FieldHash/lib/Hash/Util/FieldHash.pm b/ext/Hash/Util/FieldHash/lib/Hash/Util/FieldHash.pm
new file mode 100644
index 0000000000..cf20f55876
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/lib/Hash/Util/FieldHash.pm
@@ -0,0 +1,442 @@
+package Hash::Util::FieldHash;
+
+use 5.009004;
+use strict;
+use warnings;
+use Carp qw( croak);
+use Scalar::Util qw( reftype);
+
+require Exporter;
+our @ISA = qw(Exporter);
+our %EXPORT_TAGS = (
+    'all' => [ qw(
+        fieldhash
+        fieldhashes
+    )],
+);
+our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
+our @EXPORT = qw(
+);
+
+our $VERSION = '0.01';
+
+{
+    require XSLoader;
+    our %ob_reg; # silence possible 'once' warning in XSLoader
+    XSLoader::load('Hash::Util::FieldHash', $VERSION);
+}
+
+sub fieldhash (\%) {
+    for ( shift ) {
+        return unless ref() && reftype( $_) eq 'HASH';
+        return $_ if Hash::Util::FieldHash::_fieldhash( $_, 0);
+        return $_ if Hash::Util::FieldHash::_fieldhash( $_, 1);
+        return;
+    }
+}
+
+sub fieldhashes { map &fieldhash( $_), @_ }
+
+1;
+__END__
+
+=head1 NAME
+
+Hash::Util::FieldHash - Associate references with data
+
+=head1 SYNOPSIS
+
+  use Hash::Util qw(fieldhash fieldhashes);
+  
+  # Create a single field hash
+  fieldhash my %foo;
+  
+  # Create three at once...
+  fieldhashes \ my(%foo, %bar, %baz);
+  # ...or any number
+  fieldhashes @hashrefs;
+
+=head1 Functions
+
+Two functions generate field hashes:
+
+=over
+
+=item fieldhash
+
+    fieldhash %hash;
+
+Creates a single field hash.  The argument must be a hash.  Returns
+a reference to the given hash if successful, otherwise nothing.
+
+=item fieldhashes
+
+    fieldhashes @hashrefs;
+
+Creates any number of field hashes.  Arguments must be hash references.
+Returns the converted hashrefs in list context, their number in scalar
+context.
+
+=back
+
+=head1 Description
+
+=head2 Features
+
+Field hashes have three basic features:
+
+=over
+
+=item Key exchange
+
+If a I<reference> is used as a field hash key, it is replaced by
+the integer value of the reference address.
+
+=item Thread support
+
+In a new I<thread> a field hash is updated so that its keys reflect
+the new reference addresses of the original objects.
+
+=item Garbage collection
+
+When a reference goes I<stale> after having been used as a field hash key,
+the hash entry will be deleted.
+
+=back
+
+Field hashes are designed to maintain an association of a reference
+with a value. The association is independent of the bless status of
+the key, it is thread safe and garbage-collected.  These properties
+are desirable in the construction of inside-out classes.
+
+When used with keys that are plain scalars (not references), field
+hashes behave like normal hashes.
+
+=head2 Rationale
+
+The association of a reference (namely an object) with a value is
+central to the concept of inside-out classes.  These classes don't
+store the values of object variables (fields) inside the object itself,
+but outside, as it were, in private hashes keyed by the object.
+
+Normal hashes can be used for the purpose, but turn out to have
+some disadvantages:
+
+=over
+
+=item Stringification
+
+The stringification of references depends on the bless status of the
+reference.  A plain hash reference C<$ref> may stringify as C<HASH(0x1801018)>,
+but after being blessed into class C<foo> the same reference will look like
+as C<foo=HASH(0x1801018)>, unless class C<foo> overloads stringification,
+in which case it may show up as C<wurzelzwerg>.  In a normal hash, the
+stringified reference wouldn't be found again after the blessing.
+
+Bypassing stringification by use of C<Scalar::Util::refaddr> has been
+used to correct this.  Field hashes automatically stringify their
+keys to the reference address in decimal.
+
+=item Thread Dependency
+
+When a new thread is created, the Perl interpreter is cloned, which
+implies that all variables change their reference address.  Thus,
+in a daughter thread, the "same" reference C<$ref> contains a different
+address, but the cloned hash still holds the key based on the original
+address.  Again, the association is broken.
+
+A C<CLONE> method is required to update the hash on thread creation.
+Field hashes come with an appropriate C<CLONE>.
+
+=item Garbage Collection
+
+When a reference (an object) is used as a hash key, the entry stays
+in the hash when the object eventually goes out of scope.  That can result
+in a memory leak because the data associated with the object is not
+freed.  Worse than that, it can lead to a false association if the
+reference address of the original object is later re-used.  This
+is not a remote possibility, address re-use happens all the time and
+is a certainty under many conditions.
+
+If the references in question are indeed objects, a C<DESTROY> method
+I<must> clean up hashes that the object uses for storage.  Special
+methods are needed when unblessed references can occur.
+
+Field hashes have garbage collection built in.  If a reference
+(blessed or unblessed) goes out of scope, corresponding entries
+will be deleted from all field hashes.
+
+=back
+
+Thus, an inside-out class based on field hashes doesn't need a C<DESTROY>
+method, nor a C<CLONE> method for thread support.  That facilitates the
+construction considerably.
+
+=head2 How to use
+
+Traditionally, the definition of an inside-out class contains a bare
+block inside which a number of lexical hashes are declared and the
+basic accessor methods defined, usually through C<Scalar::Util::refaddr>.
+Further methods may be defined outside this block.  There has to be
+a DESTROY method and, for thread support, a CLONE method.
+
+When field hashes are used, the basic structure reamins the same.
+Each lexical hash will be made a field hash.  The call to C<refaddr>
+can be omitted from the accessor methods.  DESTROY and CLONE methods
+are not necessary.
+
+If you have an existing inside-out class, simply making all hashes
+field hashes with no other change should make no difference.  Through
+the calls to C<refaddr> or equivalent, the field hashes never get to
+see a reference and work like normal hashes.  Your DESTROY (and
+CLONE) methods are still needed.
+
+To make the field hashes kick in, it is easiest to redefine C<refaddr>
+as
+
+    sub refaddr { shift }
+
+instead of importing it from C<Scalar::Util>.  It should now be possible
+to disable DESTROY and CLONE.  Note that while it isn't disabled,
+DESTROY will be called before the garbage collection of field hashes,
+so it will be invoked with a functional object.
+
+It is not necessary to import the functions C<fieldhash> and/or
+C<fieldhashes> into every class that is going to use them.  When
+the class is up and running, these functions have no business there.
+If there are only a few field hashes to declare, it is simplest to
+
+    use Hash::Util::FieldHash;
+
+early and call the functions qualified:
+
+    Hash::Util::FieldHash::fieldhash my %foo;
+
+Otherwise, import the functions into a convenient package like
+C<HUF> or, more generic, C<Aux>
+
+    {
+        package Aux;
+        use Hash::Util::FieldHash ':all';
+    }
+
+and call
+
+    Aux::fieldhash my %foo;
+
+as needed.
+
+=head2 Examples
+
+Well... really only one example, and a rather trivial one at that.
+There isn't much to exemplify.
+
+=head3 A simple class...
+
+The following example shows an utterly simple inside-out class
+C<TimeStamp>, created using field hashes.  It has a single field,
+incorporated as the field hash C<%time>.  Besides C<new> it has only
+two methods: an initializer called C<stamp> that sets the field to
+the current time, and a read-only accessor C<when> that returns the
+time in C<localtime> format.
+
+    # The class TimeStamp
+
+    use Hash::Util::FieldHash;
+    {
+        package TimeStamp;
+
+        Hash::Util::FieldHash::fieldhash my %time;
+
+        sub stamp { $time{ $_[ 0]} = time; shift }       # initializer
+        sub when { scalar localtime $time{ shift()} }    # read accessor
+        sub new { bless( do { \ my $x }, shift)->stamp } # creator
+    }
+
+    # See if it works
+    my $ts = TimeStamp->new;
+    print $ts->when, "\n";
+
+Remarkable about this class definition is what isn't there: there
+is no C<DESTROY> method, inherited or local, and no C<CLONE> method
+is needed to make it thread-safe.  Not to mention no need to call
+C<refaddr> or something similar in the accessors.
+
+=head3 ...in action
+
+The outstanding property of inside-out classes is their "inheritability".
+Like all inside-out classes, C<TimeStamp> is a I<universal base class>.
+We can put it on the C<@ISA> list of arbitrary classes and its methods
+will just work, no matter how the host class is constructed.  This is
+demonstrated by the following program:
+
+    # Make a sample of objects to add time stamps to.
+
+    use Math::Complex;
+    use IO::Handle;
+
+    my @objects = (
+        Math::Complex->new( 12, 13),
+        IO::Handle->new(),
+        qr/abc/,                         # in class Regexp
+        bless( [], 'Boing'),             # made up on the spot
+    );
+
+    # Prepare for use with TimeStamp
+    
+    for ( @objects ) {
+        no strict 'refs';
+        push @{ ref() . '::ISA' }, 'TimeStamp';
+    }
+
+    # Now apply TimeStamp methods to all objects and show the result
+
+    for my $obj ( @objects ) {
+        $obj->stamp;
+        report( $obj, $obj->when);
+    }
+
+    # print a description of the object and the result of ->when
+
+    use Scalar::Util qw( reftype);
+    sub report {
+        my ( $obj, $when) = @_;
+        my $msg = sprintf "This is a %s object(a %s), its time is %s",
+            ref $obj,
+            reftype $obj,
+            $when;
+        $msg =~ s/\ba(?= [aeiouAEIOU])/an/g; # grammar matters :)
+        print "$msg\n";
+    }
+
+=head2 Garbage-Collected Hashes
+
+Garbage collection in a field hash means that entries will "spontaneously"
+disappear when the object that created them disappears.  That must be
+borne in mind, especially when looping over a field hash.  If anything
+you do inside the loop could cause an object to go out of scope, a
+random key may be deleted from the hash you are looping over.  That
+can throw the loop iterator, so it's best to cache a consistent snapshot
+of the keys and/or values and loop over that.  You will still have to
+check that a cached entry still exists when you get to it.
+
+Garbage collection can be confusing when keys are created in a field hash
+from normal scalars as well as references.  Once a reference is I<used> with
+a field hash, the entry will be collected, even if it was later overwritten
+with a plain scalar key (every positive integer is a candidate).  This
+is true even if the original entry was deleted in the meantime.  In fact,
+deletion from a field hash, and also a test for existence constitute
+I<use> in this sense and create a liability to delete the entry when
+the reference goes out of scope.  If you happen to create an entry
+with an identical key from a string or integer, that will be collected
+instead.  Thus, mixed use of references and plain scalars as field hash
+keys is not entirely supported.
+
+=head1 Guts
+
+To make C<Hash::Util::FieldHash> work, there were two changes to
+F<perl> itself.  C<PERL_MAGIC_uvar> was made avaliable for hashes,
+and weak references now call uvar C<get> magic after a weakref has been
+cleared.  The first feature is used to make field hashes intercept
+their keys upon access.  The second one triggers garbage collection.
+
+=head2 The C<PERL_MAGIC_uvar> interface for hashes
+
+C<PERL_MAGIC_uvar> I<get> magic is called from C<hv_fetch_common> and
+C<hv_delete_common> through the function C<hv_magic_uvar_xkey>, which
+defines the interface.  The call happens for hashes with "uvar" magic
+if the C<ufuncs> structure has equal values in the C<uf_val> and C<uf_set>
+fields.  Hashes are unaffected if (and as long as) these fields
+hold different values.
+
+Upon the call, the C<mg_obj> field will hold the hash key to be accessed.
+Upon return, the C<SV*> value in C<mg_obj> will be used in place of the
+original key in the hash access.  The integer index value in the first
+parameter will be the C<action> value from C<hv_fetch_common>, or -1
+if the call is from C<hv_delete_common>.
+
+This is a template for a function suitable for the C<uf_val> field in
+a C<ufuncs> structure for this call.  The C<uf_set> and C<uf_index>
+fields are irrelevant.
+
+    IV watch_key(pTHX_ IV action, SV* field) {
+        MAGIC* mg = mg_find(field, PERL_MAGIC_uvar);
+        SV* keysv = mg->mg_obj;
+        /* Do whatever you need to.  If you decide to
+           supply a different key newkey, return it like this
+        */
+        sv_2mortal(newkey);
+        mg->mg_obj = newkey;
+        return 0;
+    }
+
+=head2 Weakrefs call uvar magic
+
+When a weak reference is stored in an C<SV> that has "uvar" magic, C<set>
+magic is called after the reference has gone stale.  This hook can be
+used to trigger further garbage-collection activities associated with
+the referenced object.
+
+=head2 How field hashes work
+
+The three features of key hashes, I<key replacement>, I<thread support>,
+and I<garbage collection> are supported by a data structure called
+the I<object registry>.  This is currently the hash
+C<Hash::Utils::FieldHash::ob_reg> though there may be a more private
+place for it in the future.  An "object" is any reference (blessed
+or unblessed) that has been used as a field hash key.
+
+The object registry keeps track of references that have been used as
+field hash keys.  The keys are generated from the reference address
+like in a field hash (though the registry isn't a field hash).  Each
+value is a weak copy of the original reference, stored in an C<SV> that
+is itself magical (C<PERL_MAGIC_uvar> again).  The magical structure
+holds a list (another hash, really) of field hashes that the reference
+has been used with.  When the weakref becomes stale, the magic is
+activated and uses the list to delete the reference from all field
+hashes it has been used with.  After that, the entry is removed from
+the object registry itself.  Implicitly, that frees the magic structure
+and the storage it has been using.
+
+Whenever a reference is used as a field hash key, the object registry
+is checked and a new entry is made if necessary.  The field hash is
+then added to the list of fields this reference has used.
+
+The object registry is also used to repair a field hash after thread
+cloning.  Here, the entire object registry is processed.  For every
+reference found there, the field hashes it has used are visited and
+the entry is updated.
+
+=head2 Internal function Hash::Util::FieldHash::_fieldhash
+
+    # test if %hash is a field hash
+    my $result = _fieldhash \ %hash, 0;
+
+    # make %hash a field hash
+    my $result = _fieldhash \ %hash, 1;
+
+C<_fieldhash> is the internal function used to create field hashes.
+It takes two arguments, a hashref and a mode.  If the mode is boolean
+false, the hash is not changed but tested if it is a field hash.  If
+the hash isn't a field hash the return value is boolean false.  If it
+is, the return value indicates the mode of field hash.  When called with
+a boolean true mode, it turns the given hash into a field hash of this
+mode, returning the mode of the created field hash.  C<_fieldhash>
+does not erase the given hash.
+
+Currently there is only one type of field hash, and only the boolean
+value of the mode makes a difference, but that may change.
+
+=head1 AUTHOR
+
+Anno Siegel, E<lt>anno4000@zrz.tu-berlin.deE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright (C) 2006 by (icke)
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself, either Perl version 5.8.7 or,
+at your option, any later version of Perl 5 you may have available.
+
+=cut
diff --git a/ext/Hash/Util/FieldHash/t/01_load.t b/ext/Hash/Util/FieldHash/t/01_load.t
new file mode 100644
index 0000000000..952f2a3532
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/01_load.t
@@ -0,0 +1,59 @@
+#!perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use strict; use warnings;
+
+use Test::More tests => 8;
+
+# see that Hash::Util::FieldHash and Hash::Util load and export what
+# they should
+
+# note to self: this test only works in the perl build environment,
+# not in my homely test environment (haven't got the right Hash::Util.pm
+# there).  mask it.
+
+BEGIN {
+    use_ok( 'Hash::Util');
+    ok( defined( &Hash::Util::lock_keys), "Hash::Util::lock_keys found");
+    ok( !defined( &Hash::Util::FieldHash::fieldhashes),
+        "Hash::Util::FieldHash not loaded",
+    );
+}
+
+package one;
+use Test::More;
+use Hash::Util qw( lock_keys);
+BEGIN {
+    ok( defined( &lock_keys), "lock_keys imported from Hash::Util");
+}
+
+use Hash::Util qw( fieldhashes);
+BEGIN {
+    ok( defined( &Hash::Util::FieldHash::fieldhashes),
+        "Hash::Util::FieldHash loaded",
+    );
+    ok( defined( &fieldhashes),
+        "fieldhashes imported from Hash::Util",
+    );
+}
+
+package two;
+use Test::More;
+use Hash::Util::FieldHash qw( fieldhashes);
+BEGIN {
+    ok( defined( &fieldhashes),
+        "fieldhashes imported from Hash::Util::FieldHash",
+    );
+}
+
+use Hash::Util::FieldHash qw( :all);
+BEGIN {
+    ok( defined( &fieldhash),
+        "fieldhash imported from Hash::Util::FieldHash via :all",
+    );
+}
+
diff --git a/ext/Hash/Util/FieldHash/t/02_function.t b/ext/Hash/Util/FieldHash/t/02_function.t
new file mode 100644
index 0000000000..a89bf2e92b
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/02_function.t
@@ -0,0 +1,215 @@
+#!perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use strict; use warnings;
+use Test::More;
+my $n_tests = 0;
+
+use Hash::Util::FieldHash qw( :all);
+
+#########################
+
+# define ref types to use with some tests
+my @test_types;
+BEGIN {
+    # skipping CODE refs, they are differently scoped
+    @test_types = qw( SCALAR ARRAY HASH GLOB);
+}
+
+### Object registry
+
+BEGIN { $n_tests += 3 }
+{
+    my $ob_reg = \ %Hash::Util::FieldHash::ob_reg;
+    {
+        my $obj = {};
+        {
+            my $h;
+            fieldhash %$h;
+            $h->{ $obj} = 123;
+            is( keys %$ob_reg, 1, "one object registered");
+        }
+        # field hash stays alive until $obj dies
+        is( keys %$ob_reg, 1, "object still registered");
+    }
+    is( keys %$ob_reg, 0, "object unregistered");
+}
+
+### existence/retrieval/deletion
+BEGIN { $n_tests += 6 }
+{
+    no warnings 'misc';
+    my $val = 123;
+    fieldhash my %h;
+    for ( [ str => 'abc'], [ ref => {}] ) {
+        my ( $keytype, $key) = @$_;
+        $h{ $key} = $val;
+        ok( exists $h{ $key},  "existence ($keytype)");
+        is( $h{ $key}, $val,   "retrieval ($keytype)");
+        delete $h{ $key};
+        is( keys %h, 0, "deletion ($keytype)");
+    }
+}
+
+### id-action (stringification independent of bless)
+BEGIN { $n_tests += 4 }
+{
+    my( %f, %g, %h, %i);
+    fieldhash %f;
+    fieldhash %g;
+    my $val = 123;
+    my $key = [];
+    $f{ $key} = $val;
+    is( $f{ $key}, $val, "plain key set in field");
+    bless $key;
+    is( $f{ $key}, $val, "access through blessed");
+    $key = [];
+    $h{ $key} = $val;
+    is( $h{ $key}, $val, "plain key set in hash");
+    bless $key;
+    isnt( $h{ $key}, $val, "no access through blessed");
+}
+    
+# Garbage collection
+BEGIN { $n_tests += 1 + 2*( 3*@test_types + 5) + 1 }
+
+{
+    fieldhash my %h;
+    $h{ []} = 123;
+    is( keys %h, 0, "blip");
+}
+
+for my $preload ( [], [ map {}, 1 .. 3] ) {
+    my $pre = @$preload ? ' (preloaded)' : '';
+    fieldhash my %f;
+    my @preval = map "$_", @$preload;
+    @f{ @$preload} = @preval;
+    # Garbage collection separately
+    for my $type ( @test_types) {
+        {
+            my $ref = gen_ref( $type);
+            $f{ $ref} = $type;
+            my ( $val) = grep $_ eq $type, values %f;
+            is( $val, $type, "$type visible$pre");
+            is( 
+                keys %Hash::Util::FieldHash::ob_reg,
+                1 + @$preload,
+                "$type obj registered$pre"
+            );
+        }
+        is( keys %f, @$preload, "$type gone$pre");
+    }
+    
+    # Garbage collection collectively
+    is( keys %Hash::Util::FieldHash::ob_reg, @$preload, "no objs remaining$pre");
+    {
+        my @refs = map gen_ref( $_), @test_types;
+        @f{ @refs} = @test_types;
+        ok(
+            eq_set( [ values %f], [ @test_types, @preval]),
+            "all types present$pre",
+        );
+        is(
+            keys %Hash::Util::FieldHash::ob_reg,
+            @test_types + @$preload,
+            "all types registered$pre",
+        );
+    }
+    die "preload gone" unless defined $preload;
+    ok( eq_set( [ values %f], \ @preval), "all types gone$pre");
+    is( keys %Hash::Util::FieldHash::ob_reg, @$preload, "all types unregistered$pre");
+}
+is( keys %Hash::Util::FieldHash::ob_reg, 0, "preload gone after loop");
+
+# big key sets
+BEGIN { $n_tests += 8 }
+{
+    my $size = 10_000;
+    fieldhash( my %f);
+    {
+        my @refs = map [], 1 .. $size;
+        $f{ $_} = 1 for @refs;
+        is( keys %f, $size, "many keys singly");
+        is(
+            keys %Hash::Util::FieldHash::ob_reg,
+            $size,
+            "many objects singly",
+        );
+    }
+    is( keys %f, 0, "many keys singly gone");
+    is(
+        keys %Hash::Util::FieldHash::ob_reg,
+        0,
+        "many objects singly unregistered",
+    );
+    
+    {
+        my @refs = map [], 1 .. $size;
+        $f{ $_} = 1 for @refs;
+        is( keys %f, $size, "many keys at once");
+        is(
+            keys %Hash::Util::FieldHash::ob_reg,
+            $size,
+            "many objects at once",
+        );
+    }
+    is( keys %f, 0, "many keys at once gone");
+    is(
+        keys %Hash::Util::FieldHash::ob_reg,
+        0,
+        "many objects at once unregistered",
+    );
+}
+
+# many field hashes
+BEGIN { $n_tests += 6 }
+{
+    my $n_fields = 1000;
+    my @fields = map &fieldhash( {}), 1 .. $n_fields;
+    my @obs = map gen_ref( $_), @test_types;
+    my $n_obs = @obs;
+    for my $field ( @fields ) {
+        @{ $field }{ @obs} = map ref, @obs;
+    }
+    my $err = grep keys %$_ != @obs, @fields;
+    is( $err, 0, "$n_obs entries in $n_fields fields");
+    is( keys %Hash::Util::FieldHash::ob_reg, @obs, "$n_obs obs registered");
+    pop @obs;
+    $err = grep keys %$_ != @obs, @fields;
+    is( $err, 0, "one entry gone from $n_fields fields");
+    is( keys %Hash::Util::FieldHash::ob_reg, @obs, "one ob unregistered");
+    @obs = ();
+    $err = grep keys %$_ != @obs, @fields;
+    is( $err, 0, "all entries gone from $n_fields fields");
+    is( keys %Hash::Util::FieldHash::ob_reg, @obs, "all obs unregistered");
+}
+
+{
+
+    BEGIN { $n_tests += 1 }
+    fieldhash my %h;
+    bless \ %h, 'abc'; # this bus-errors with a certain bug
+    ok( 1, "no bus error on bless")
+}
+
+BEGIN { plan tests => $n_tests }
+
+#######################################################################
+
+use Symbol qw( gensym);
+
+BEGIN {
+    my %gen = (
+        SCALAR => sub { \ my $x },
+        ARRAY  => sub { [] },
+        HASH   => sub { {} },
+        GLOB   => sub { gensym },
+        CODE   => sub { sub {} },
+    );
+
+    sub gen_ref { $gen{ shift()}->() }
+}
diff --git a/ext/Hash/Util/FieldHash/t/03_class.t b/ext/Hash/Util/FieldHash/t/03_class.t
new file mode 100644
index 0000000000..027b43c198
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/03_class.t
@@ -0,0 +1,116 @@
+#!perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use strict; use warnings;
+use Test::More;
+my $n_tests = 0;
+
+use Config;
+BEGIN { $n_tests += 2 }
+{
+    my $p = Impostor->new( 'Donald Duck');
+    is( $p->greeting, "Hi, I'm Donald Duck", "blank title");
+    $p->assume_title( 'Mr');
+    is( $p->greeting, "Hi, I'm Mr Donald Duck", "changed title");
+}
+
+# thread support?
+BEGIN { $n_tests += 5 }
+SKIP: {
+    skip "No thread support", 5 unless $Config{ usethreads};
+    require threads;
+    treads->import if threads->can( 'import');
+
+    my $ans;
+    my $p = Impostor->new( 'Donald Duck');
+    $ans = threads->create( sub { $p->greeting })->join;
+    is( $ans, "Hi, I'm Donald Duck", "thread: blank title");
+    $p->assume_title( 'Mr');
+    $ans = threads->create( sub { $p->greeting })->join;
+    is( $ans, "Hi, I'm Mr Donald Duck", "thread: changed title");
+    $ans = threads->create(
+        sub {
+            $p->assume_title( 'Uncle');
+            $p->greeting;
+        }
+    )->join;
+    is( $ans, "Hi, I'm Uncle Donald Duck", "thread: local change");
+    is( $p->greeting, "Hi, I'm Mr Donald Duck", "thread: change is local");
+
+    # second generation thread
+    $ans = threads->create(
+        sub {
+            threads->create( sub { $p->greeting })->join;
+        }
+    )->join;
+    is( $ans, "Hi, I'm Mr Donald Duck", "double thread: got greeting");
+}
+
+BEGIN { plan tests => $n_tests }
+
+############################################################################
+
+# must do this in BEGIN so that field hashes are declared before
+# first use above
+
+BEGIN {
+    package CFF;
+    use Hash::Util::FieldHash qw( :all);
+
+    package Person;
+
+    {
+        CFF::fieldhash my %name;
+        CFF::fieldhash my %title;
+
+        sub init {
+            my $p = shift;
+            $name{ $p} = shift || '';
+            $title{ $p} = shift || '';
+            $p;
+        }
+
+        sub name { $name{ shift()} }
+        sub title { $title{ shift() } }
+    }
+
+    sub new {
+        my $class = shift;
+        bless( \ my $x, $class)->init( @_);
+    }
+
+    sub greeting {
+        my $p = shift;
+        my $greet = "Hi, I'm ";
+        $_ and $greet .= "$_ " for $p->title;
+        $greet .= $p->name;
+        $greet;
+    }
+
+    package Impostor;
+    use base 'Person';
+
+    {
+        CFF::fieldhash my %assumed_title;
+
+        sub init {
+            my $p = shift;
+            my ( $name, $title) = @_;
+            $p->Person::init( $name, $title);
+            $p->assume_title( $title);
+            $p;
+        }
+
+        sub title { $assumed_title{ shift()} }
+        
+        sub assume_title {
+            my $p = shift;
+            $assumed_title{ $p} = shift || '';
+            $p;
+        }
+    }
+}
diff --git a/ext/Hash/Util/FieldHash/t/04_thread.t b/ext/Hash/Util/FieldHash/t/04_thread.t
new file mode 100644
index 0000000000..0693522181
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/04_thread.t
@@ -0,0 +1,68 @@
+#!perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use strict; use warnings;
+use Test::More;
+my $n_tests;
+
+use Hash::Util::FieldHash qw( :all);
+
+{
+    my $n_basic;
+    BEGIN {
+        $n_basic = 6; # 6 tests per call of basic_func()
+        $n_tests += 5*$n_basic;
+    }
+    my $ob_reg = \ %Hash::Util::FieldHash::ob_reg;
+    my %h;
+    fieldhash %h;
+
+    sub basic_func {
+        my $level = shift;
+        
+        my @res;
+        my $push_is = sub {
+            my ( $hash, $should, $name) = @_;
+            push @res, [ scalar keys %$hash, $should, $name];
+        };
+            
+        my $obj = [];
+        $push_is->( \ %h, 0, "$level: initially clear");
+        $push_is->( $ob_reg, 0, "$level: ob_reg initially clear");
+        $h{ $obj} = 123;
+        $push_is->( \ %h, 1, "$level: one object");
+        $push_is->( $ob_reg, 1, "$level: ob_reg one object");
+        undef $obj;
+        $push_is->( \ %h, 0, "$level: garbage collected");
+        $push_is->( $ob_reg, 0, "$level: ob_reg garbage collected");
+        @res;
+    }
+
+    &is( @$_) for basic_func( "home");
+
+    SKIP: {
+        require Config;
+        skip "No thread support", 3*$n_basic unless
+            $Config::Config{ usethreads};
+        require threads;
+        my ( $t) = threads->create( \ &basic_func, "thread 1");
+        &is( @$_) for $t->join;
+
+        &is( @$_) for basic_func( "back home");
+
+        ( $t) = threads->create( sub {
+            my ( $t) = threads->create( \ &basic_func, "thread 2");
+            $t->join;
+        });
+        &is( @$_) for $t->join;
+    }
+
+    &is( @$_) for basic_func( "back home again");
+
+}
+
+BEGIN { plan tests => $n_tests }
diff --git a/ext/Hash/Util/FieldHash/t/05_perlhook.t b/ext/Hash/Util/FieldHash/t/05_perlhook.t
new file mode 100644
index 0000000000..73f8654118
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/05_perlhook.t
@@ -0,0 +1,174 @@
+#!perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use strict; use warnings;
+use Test::More;
+my $n_tests;
+
+use Hash::Util::FieldHash;
+use Scalar::Util qw( weaken);
+
+# The functions in Hash::Util::FieldHash
+# _test_uvar_get, _test_uvar_get and _test_uvar_both
+
+# _test_uvar_get( $anyref, \ $counter) makes the referent of $anyref
+# "uvar"-magical with get magic only.  $counter is reset if the magic
+# could be established.  $counter will be incremented each time the
+# magic "get" function is called.
+
+# _test_uvar_set does the same for "set" magic.  _test_uvar_both
+# sets both magic functions identically.  Both use the same counter.
+
+# magical weak ref (patch to sv.c)
+{
+    my( $magref, $counter);
+
+    $counter = 123;
+    Hash::Util::FieldHash::_test_uvar_set( \ $magref, \ $counter);
+    is( $counter, 0, "got magical scalar");
+
+    my $ref = [];
+    $magref = $ref;
+    is( $counter, 1, "store triggers magic");
+
+    weaken $magref;
+    is( $counter, 1, "weaken doesn't trigger magic");
+    
+    { my $x = $magref }
+    is( $counter, 1, "read doesn't trigger magic");
+
+    undef $ref;
+    is( $counter, 2, "ref expiry triggers magic (weakref patch worked)");
+
+    is( $magref, undef, "weak ref works normally");
+
+    # same, but overwrite weakref before expiry
+    $counter = 0;
+    weaken( $magref = $ref = []);
+    is( $counter, 1, "setup for overwrite");
+
+    $magref = my $other_ref = [];
+    is( $counter, 2, "overwrite triggers");
+    
+    undef $ref;
+    is( $counter, 2, "ref expiry doesn't trigger after overwrite");
+
+    is( $magref, $other_ref, "weak ref doesn't kill overwritten value");
+
+    BEGIN { $n_tests += 10 }
+}
+
+# magical hash (patches to mg.c and hv.c)
+{
+    # the hook is only sensitive if the set function is NULL
+    my ( %h, $counter);
+    $counter = 123;
+    Hash::Util::FieldHash::_test_uvar_get( \ %h, \ $counter);
+    is( $counter, 0, "got magical hash");
+
+    %h = ( abc => 123);
+    is( $counter, 1, "list assign triggers");
+
+    $h{ def} = 456;
+    is( $counter, 3, "lvalue assign triggers twice");
+
+    exists $h{ def};
+    is( $counter, 4, "good exists triggers");
+
+    exists $h{ xyz};
+    is( $counter, 5, "bad exists triggers");
+
+    delete $h{ def};
+    is( $counter, 6, "good delete triggers");
+
+    delete $h{ xyz};
+    is( $counter, 7, "bad delete triggers");
+
+    my $x = $h{ abc};
+    is( $counter, 8, "good read triggers");
+
+    $x = $h{ xyz};
+    is( $counter, 9, "bad read triggers");
+
+    bless \ %h;
+    is( $counter, 9, "bless triggers(!)");
+
+
+    $x = keys %h;
+    is( $counter, 9, "scalar keys doesn't trigger");
+
+    () = keys %h;
+    is( $counter, 9, "list keys doesn't trigger");
+
+    $x = values %h;
+    is( $counter, 9, "scalar values doesn't trigger");
+
+    () = values %h;
+    is( $counter, 9, "list values doesn't trigger");
+
+    $x = each %h;
+    is( $counter, 9, "scalar each doesn't trigger");
+
+    () = each %h;
+    is( $counter, 9, "list each doesn't trigger");
+
+    # see that normal set magic doesn't trigger (identity condition)
+    my %i;
+    Hash::Util::FieldHash::_test_uvar_set( \ %i, \ $counter);
+    is( $counter, 0, "got magical hash");
+
+    %i = ( abc => 123);
+    $i{ def} = 456;
+    exists $i{ def};
+    exists $i{ xyz};
+    delete $i{ def};
+    delete $i{ xyz};
+    $x = $i{ abc};
+    $x = $i{ xyz};
+    $x = keys %i;
+    () = keys %i;
+    $x = values %i;
+    () = values %i;
+    $x = each %i;
+    () = each %i;
+    
+    is( $counter, 0, "normal set magic never triggers");
+
+    bless \ %i, 'abc';
+    is( $counter, 1, "...except with bless");
+
+    # see that magic with both set and get doesn't trigger (identity condition)
+    $counter = 123;
+    my %j;
+    Hash::Util::FieldHash::_test_uvar_same( \ %j, \ $counter);
+    is( $counter, 0, "got magical hash");
+
+    %j = ( abc => 123);
+    $j{ def} = 456;
+    exists $j{ def};
+    exists $j{ xyz};
+    delete $j{ def};
+    delete $j{ xyz};
+    $x = $j{ abc};
+    $x = $j{ xyz};
+    $x = keys %j;
+    () = keys %j;
+    $x = values %j;
+    () = values %j;
+    $x = each %j;
+    () = each %j;
+    
+    is( $counter, 0, "normal get magic never triggers");
+
+    bless \ %j, 'abc';
+    is( $counter, 1, "...except for bless");
+
+    BEGIN { $n_tests += 22 }
+}
+
+BEGIN { plan tests => $n_tests }
+
diff --git a/ext/Hash/Util/FieldHash/t/10_hash.t b/ext/Hash/Util/FieldHash/t/10_hash.t
new file mode 100644
index 0000000000..80de72291d
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/10_hash.t
@@ -0,0 +1,116 @@
+#!./perl -w
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use Test::More;
+
+use strict;
+use Hash::Util::FieldHash qw( :all);
+
+no warnings 'misc';
+
+plan tests => 5;
+
+fieldhash my %h;
+
+ok (!Internals::HvREHASH(%h), "hash doesn't start with rehash flag on");
+
+foreach (1..10) {
+  $h{"\0"x$_}++;
+}
+
+ok (!Internals::HvREHASH(%h), "10 entries doesn't trigger rehash");
+
+foreach (11..20) {
+  $h{"\0"x$_}++;
+}
+
+ok (Internals::HvREHASH(%h), "20 entries triggers rehash");
+
+
+
+
+# second part using an emulation of the PERL_HASH in perl, mounting an
+# attack on a prepopulated hash. This is also useful if you need normal
+# keys which don't contain \0 -- suitable for stashes
+
+use constant MASK_U32  => 2**32;
+use constant HASH_SEED => 0;
+use constant THRESHOLD => 14;
+use constant START     => "a";
+
+# some initial hash data
+fieldhash my %h2;
+%h2 = map {$_ => 1} 'a'..'cc';
+
+ok (!Internals::HvREHASH(%h2), 
+    "starting with pre-populated non-pathalogical hash (rehash flag if off)");
+
+my @keys = get_keys(\%h2);
+$h2{$_}++ for @keys;
+ok (Internals::HvREHASH(%h2), 
+    scalar(@keys) . " colliding into the same bucket keys are triggerring rehash");
+
+sub get_keys {
+    my $hr = shift;
+
+    # the minimum of bits required to mount the attack on a hash
+    my $min_bits = log(THRESHOLD)/log(2);
+
+    # if the hash has already been populated with a significant amount
+    # of entries the number of mask bits can be higher
+    my $keys = scalar keys %$hr;
+    my $bits = $keys ? log($keys)/log(2) : 0;
+    $bits = $min_bits if $min_bits > $bits;
+
+    $bits = int($bits) < $bits ? int($bits) + 1 : int($bits);
+    # need to add 2 bits to cover the internal split cases
+    $bits += 2;
+    my $mask = 2**$bits-1;
+    print "# using mask: $mask ($bits)\n";
+
+    my @keys;
+    my $s = START;
+    my $c = 0;
+    # get 2 keys on top of the THRESHOLD
+    my $hash;
+    while (@keys < THRESHOLD+2) {
+        # next if exists $hash->{$s};
+        $hash = hash($s);
+        next unless ($hash & $mask) == 0;
+        $c++;
+        printf "# %2d: %5s, %10s\n", $c, $s, $hash;
+        push @keys, $s;
+    } continue {
+        $s++;
+    }
+
+    return @keys;
+}
+
+
+# trying to provide the fastest equivalent of C macro's PERL_HASH in
+# Perl - the main complication is that it uses U32 integer, which we
+# can't do it perl, without doing some tricks
+sub hash {
+    my $s = shift;
+    my @c = split //, $s;
+    my $u = HASH_SEED;
+    for (@c) {
+        # (A % M) + (B % M) == (A + B) % M
+        # This works because '+' produces a NV, which is big enough to hold
+        # the intermidiate result. We only need the % before any "^" and "&"
+        # to get the result in the range for an I32.
+        # and << doesn't work on NV, so using 1 << 10
+        $u += ord;
+        $u += $u * (1 << 10); $u %= MASK_U32;
+        $u ^= $u >> 6;
+    }
+    $u += $u << 3;  $u %= MASK_U32;
+    $u ^= $u >> 11; $u %= MASK_U32;
+    $u += $u << 15; $u %= MASK_U32;
+    $u;
+}
diff --git a/ext/Hash/Util/FieldHash/t/11_hashassign.t b/ext/Hash/Util/FieldHash/t/11_hashassign.t
new file mode 100644
index 0000000000..205f36ebcb
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/11_hashassign.t
@@ -0,0 +1,319 @@
+#!./perl -w
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use Test::More;
+
+# use strict;
+use Hash::Util::FieldHash qw( :all);
+no warnings 'misc';
+
+plan tests => 215;
+
+my @comma = ("key", "value");
+
+# The peephole optimiser already knows that it should convert the string in
+# $foo{string} into a shared hash key scalar. It might be worth making the
+# tokeniser build the LHS of => as a shared hash key scalar too.
+# And so there's the possiblility of it going wrong
+# And going right on 8 bit but wrong on utf8 keys.
+# And really we should also try utf8 literals in {} and => in utf8.t
+
+# Some of these tests are (effectively) duplicated in each.t
+fieldhash my %comma;
+%comma = @comma;
+ok (keys %comma == 1, 'keys on comma hash');
+ok (values %comma == 1, 'values on comma hash');
+# defeat any tokeniser or optimiser cunning
+my $key = 'ey';
+is ($comma{"k" . $key}, "value", 'is key present? (unoptimised)');
+# now with cunning:
+is ($comma{key}, "value", 'is key present? (maybe optimised)');
+#tokeniser may treat => differently.
+my @temp = (key=>undef);
+is ($comma{$temp[0]}, "value", 'is key present? (using LHS of =>)');
+
+@temp = %comma;
+ok (eq_array (\@comma, \@temp), 'list from comma hash');
+
+@temp = each %comma;
+ok (eq_array (\@comma, \@temp), 'first each from comma hash');
+@temp = each %comma;
+ok (eq_array ([], \@temp), 'last each from comma hash');
+
+my %temp = %comma;
+
+ok (keys %temp == 1, 'keys on copy of comma hash');
+ok (values %temp == 1, 'values on copy of comma hash');
+is ($temp{'k' . $key}, "value", 'is key present? (unoptimised)');
+# now with cunning:
+is ($temp{key}, "value", 'is key present? (maybe optimised)');
+@temp = (key=>undef);
+is ($comma{$temp[0]}, "value", 'is key present? (using LHS of =>)');
+
+@temp = %temp;
+ok (eq_array (\@temp, \@temp), 'list from copy of comma hash');
+
+@temp = each %temp;
+ok (eq_array (\@temp, \@temp), 'first each from copy of comma hash');
+@temp = each %temp;
+ok (eq_array ([], \@temp), 'last each from copy of comma hash');
+
+my @arrow = (Key =>"Value");
+
+fieldhash my %arrow;
+%arrow = @arrow;
+ok (keys %arrow == 1, 'keys on arrow hash');
+ok (values %arrow == 1, 'values on arrow hash');
+# defeat any tokeniser or optimiser cunning
+$key = 'ey';
+is ($arrow{"K" . $key}, "Value", 'is key present? (unoptimised)');
+# now with cunning:
+is ($arrow{Key}, "Value", 'is key present? (maybe optimised)');
+#tokeniser may treat => differently.
+@temp = ('Key', undef);
+is ($arrow{$temp[0]}, "Value", 'is key present? (using LHS of =>)');
+
+@temp = %arrow;
+ok (eq_array (\@arrow, \@temp), 'list from arrow hash');
+
+@temp = each %arrow;
+ok (eq_array (\@arrow, \@temp), 'first each from arrow hash');
+@temp = each %arrow;
+ok (eq_array ([], \@temp), 'last each from arrow hash');
+
+%temp = %arrow;
+
+ok (keys %temp == 1, 'keys on copy of arrow hash');
+ok (values %temp == 1, 'values on copy of arrow hash');
+is ($temp{'K' . $key}, "Value", 'is key present? (unoptimised)');
+# now with cunning:
+is ($temp{Key}, "Value", 'is key present? (maybe optimised)');
+@temp = ('Key', undef);
+is ($arrow{$temp[0]}, "Value", 'is key present? (using LHS of =>)');
+
+@temp = %temp;
+ok (eq_array (\@temp, \@temp), 'list from copy of arrow hash');
+
+@temp = each %temp;
+ok (eq_array (\@temp, \@temp), 'first each from copy of arrow hash');
+@temp = each %temp;
+ok (eq_array ([], \@temp), 'last each from copy of arrow hash');
+
+fieldhash my %direct;
+fieldhash my %slow;
+%direct = ('Camel', 2, 'Dromedary', 1);
+$slow{Dromedary} = 1;
+$slow{Camel} = 2;
+
+ok (eq_hash (\%slow, \%direct), "direct list assignment to hash");
+%direct = (Camel => 2, 'Dromedary' => 1);
+ok (eq_hash (\%slow, \%direct), "direct list assignment to hash using =>");
+
+$slow{Llama} = 0; # A llama is not a camel :-)
+ok (!eq_hash (\%direct, \%slow), "different hashes should not be equal!");
+
+my (%names, %names_copy);
+fieldhash %names;
+%names = ('$' => 'Scalar', '@' => 'Array', # Grr '
+          '%', 'Hash', '&', 'Code');
+%names_copy = %names;
+ok (eq_hash (\%names, \%names_copy), "check we can copy our hash");
+
+sub in {
+  my %args = @_;
+  return eq_hash (\%names, \%args);
+}
+
+ok (in (%names), "pass hash into a method");
+
+sub in_method {
+  my $self = shift;
+  my %args = @_;
+  return eq_hash (\%names, \%args);
+}
+
+ok (main->in_method (%names), "pass hash into a method");
+
+sub out {
+  return %names;
+}
+%names_copy = out ();
+
+ok (eq_hash (\%names, \%names_copy), "pass hash from a subroutine");
+
+sub out_method {
+  my $self = shift;
+  return %names;
+}
+%names_copy = main->out_method ();
+
+ok (eq_hash (\%names, \%names_copy), "pass hash from a method");
+
+sub in_out {
+  my %args = @_;
+  return %args;
+}
+%names_copy = in_out (%names);
+
+ok (eq_hash (\%names, \%names_copy), "pass hash to and from a subroutine");
+
+sub in_out_method {
+  my $self = shift;
+  my %args = @_;
+  return %args;
+}
+%names_copy = main->in_out_method (%names);
+
+ok (eq_hash (\%names, \%names_copy), "pass hash to and from a method");
+
+my %names_copy2 = %names;
+ok (eq_hash (\%names, \%names_copy2), "check copy worked");
+
+# This should get ignored.
+%names_copy = ('%', 'Associative Array', %names);
+
+ok (eq_hash (\%names, \%names_copy), "duplicates at the start of a list");
+
+# This should not
+%names_copy = ('*', 'Typeglob', %names);
+
+$names_copy2{'*'} = 'Typeglob';
+ok (eq_hash (\%names_copy, \%names_copy2), "duplicates at the end of a list");
+
+%names_copy = ('%', 'Associative Array', '*', 'Endangered species', %names,
+              '*', 'Typeglob',);
+
+ok (eq_hash (\%names_copy, \%names_copy2), "duplicates at both ends");
+
+# And now UTF8
+
+foreach my $chr (60, 200, 600, 6000, 60000) {
+  # This little game may set a UTF8 flag internally. Or it may not. :-)
+  my ($key, $value) = (chr ($chr) . "\x{ABCD}", "$chr\x{ABCD}");
+  chop ($key, $value);
+  my @utf8c = ($key, $value);
+  fieldhash my %utf8c;
+  %utf8c = @utf8c;
+
+  ok (keys %utf8c == 1, 'keys on utf8 comma hash');
+  ok (values %utf8c == 1, 'values on utf8 comma hash');
+  # defeat any tokeniser or optimiser cunning
+  is ($utf8c{"" . $key}, $value, 'is key present? (unoptimised)');
+  my $tempval = sprintf '$utf8c{"\x{%x}"}', $chr;
+  is (eval $tempval, $value, "is key present? (maybe $tempval is optimised)");
+  $tempval = sprintf '@temp = ("\x{%x}" => undef)', $chr;
+  eval $tempval or die "'$tempval' gave $@";
+  is ($utf8c{$temp[0]}, $value, 'is key present? (using LHS of $tempval)');
+
+  @temp = %utf8c;
+  ok (eq_array (\@utf8c, \@temp), 'list from utf8 comma hash');
+
+  @temp = each %utf8c;
+  ok (eq_array (\@utf8c, \@temp), 'first each from utf8 comma hash');
+  @temp = each %utf8c;
+  ok (eq_array ([], \@temp), 'last each from utf8 comma hash');
+
+  %temp = %utf8c;
+
+  ok (keys %temp == 1, 'keys on copy of utf8 comma hash');
+  ok (values %temp == 1, 'values on copy of utf8 comma hash');
+  is ($temp{"" . $key}, $value, 'is key present? (unoptimised)');
+  $tempval = sprintf '$temp{"\x{%x}"}', $chr;
+  is (eval $tempval, $value, "is key present? (maybe $tempval is optimised)");
+  $tempval = sprintf '@temp = ("\x{%x}" => undef)', $chr;
+  eval $tempval or die "'$tempval' gave $@";
+  is ($temp{$temp[0]}, $value, "is key present? (using LHS of $tempval)");
+
+  @temp = %temp;
+  ok (eq_array (\@temp, \@temp), 'list from copy of utf8 comma hash');
+
+  @temp = each %temp;
+  ok (eq_array (\@temp, \@temp), 'first each from copy of utf8 comma hash');
+  @temp = each %temp;
+  ok (eq_array ([], \@temp), 'last each from copy of utf8 comma hash');
+
+  my $assign = sprintf '("\x{%x}" => "%d")', $chr, $chr;
+  print "# $assign\n";
+  my (@utf8a) = eval $assign;
+
+  fieldhash my %utf8a;
+  %utf8a = @utf8a;
+  ok (keys %utf8a == 1, 'keys on utf8 arrow hash');
+  ok (values %utf8a == 1, 'values on utf8 arrow hash');
+  # defeat any tokeniser or optimiser cunning
+  is ($utf8a{$key . ""}, $value, 'is key present? (unoptimised)');
+  $tempval = sprintf '$utf8a{"\x{%x}"}', $chr;
+  is (eval $tempval, $value, "is key present? (maybe $tempval is optimised)");
+  $tempval = sprintf '@temp = ("\x{%x}" => undef)', $chr;
+  eval $tempval or die "'$tempval' gave $@";
+  is ($utf8a{$temp[0]}, $value, "is key present? (using LHS of $tempval)");
+
+  @temp = %utf8a;
+  ok (eq_array (\@utf8a, \@temp), 'list from utf8 arrow hash');
+
+  @temp = each %utf8a;
+  ok (eq_array (\@utf8a, \@temp), 'first each from utf8 arrow hash');
+  @temp = each %utf8a;
+  ok (eq_array ([], \@temp), 'last each from utf8 arrow hash');
+
+  %temp = %utf8a;
+
+  ok (keys %temp == 1, 'keys on copy of utf8 arrow hash');
+  ok (values %temp == 1, 'values on copy of utf8 arrow hash');
+  is ($temp{'' . $key}, $value, 'is key present? (unoptimised)');
+  $tempval = sprintf '$temp{"\x{%x}"}', $chr;
+  is (eval $tempval, $value, "is key present? (maybe $tempval is optimised)");
+  $tempval = sprintf '@temp = ("\x{%x}" => undef)', $chr;
+  eval $tempval or die "'$tempval' gave $@";
+  is ($temp{$temp[0]}, $value, "is key present? (using LHS of $tempval)");
+
+  @temp = %temp;
+  ok (eq_array (\@temp, \@temp), 'list from copy of utf8 arrow hash');
+
+  @temp = each %temp;
+  ok (eq_array (\@temp, \@temp), 'first each from copy of utf8 arrow hash');
+  @temp = each %temp;
+  ok (eq_array ([], \@temp), 'last each from copy of utf8 arrow hash');
+
+}
+
+# now some tests for hash assignment in scalar and list context with
+# duplicate keys [perl #24380]
+{
+    my %h; my $x; my $ar;
+    fieldhash %h;
+    is( (join ':', %h = (1) x 8), '1:1',
+	'hash assignment in list context removes duplicates' );
+    is( scalar( %h = (1,2,1,3,1,4,1,5) ), 2,
+	'hash assignment in scalar context' );
+    is( scalar( ($x,%h) = (0,1,2,1,3,1,4,1,5) ), 3,
+	'scalar + hash assignment in scalar context' );
+    $ar = [ %h = (1,2,1,3,1,4,1,5) ];
+    is( $#$ar, 1, 'hash assignment in list context' );
+    is( "@$ar", "1 5", '...gets the last values' );
+    $ar = [ ($x,%h) = (0,1,2,1,3,1,4,1,5) ];
+    is( $#$ar, 2, 'scalar + hash assignment in list context' );
+    is( "@$ar", "0 1 5", '...gets the last values' );
+}
+
+# test stringification of keys
+{
+    no warnings 'once', 'misc';
+    my @types = qw( SCALAR         ARRAY HASH CODE    GLOB);
+    my @refs =    ( \ do { my $x }, [],   {},  sub {}, \ *x);
+    my(%h, %expect);
+    fieldhash %h;
+    @h{@refs} = @types;
+    @expect{map "$_", @refs} = @types;
+    ok (!eq_hash(\%h, \%expect), 'unblessed ref stringification different');
+
+    bless $_ for @refs;
+    %h = (); %expect = ();
+    @h{@refs} = @types;
+    @expect{map "$_", @refs} = @types;
+    ok (!eq_hash(\%h, \%expect), 'blessed ref stringification different');
+}
diff --git a/ext/Hash/Util/FieldHash/t/12_hashwarn.t b/ext/Hash/Util/FieldHash/t/12_hashwarn.t
new file mode 100644
index 0000000000..94509d25e1
--- /dev/null
+++ b/ext/Hash/Util/FieldHash/t/12_hashwarn.t
@@ -0,0 +1,60 @@
+#!./perl
+
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = '../lib';
+}
+
+use Test::More;
+
+plan( tests => 12 );
+
+use strict;
+use warnings;
+use Hash::Util::FieldHash qw( :all);
+
+use vars qw{ @warnings };
+
+BEGIN {
+    $SIG{'__WARN__'} = sub { push @warnings, @_ };
+    $| = 1;
+}
+
+my $fail_odd      = 'Odd number of elements in hash assignment at ';
+my $fail_odd_anon = 'Odd number of elements in anonymous hash at ';
+my $fail_ref      = 'Reference found where even-sized list expected at ';
+my $fail_not_hr   = 'Not a HASH reference at ';
+
+{
+    @warnings = ();
+    fieldhash my %hash;
+    %hash = (1..3);
+    cmp_ok(scalar(@warnings),'==',1,'odd count');
+    cmp_ok(substr($warnings[0],0,length($fail_odd)),'eq',$fail_odd,'odd  msg');
+
+    @warnings = ();
+    %hash = 1;
+    cmp_ok(scalar(@warnings),'==',1,'scalar count');
+    cmp_ok(substr($warnings[0],0,length($fail_odd)),'eq',$fail_odd,'scalar msg');
+
+    @warnings = ();
+    %hash = { 1..3 };
+    cmp_ok(scalar(@warnings),'==',2,'odd hashref count');
+    cmp_ok(substr($warnings[0],0,length($fail_odd_anon)),'eq',$fail_odd_anon,'odd hashref msg 1');
+    cmp_ok(substr($warnings[1],0,length($fail_ref)),'eq',$fail_ref,'odd hashref msg 2');
+
+    @warnings = ();
+    %hash = [ 1..3 ];
+    cmp_ok(scalar(@warnings),'==',1,'arrayref count');
+    cmp_ok(substr($warnings[0],0,length($fail_ref)),'eq',$fail_ref,'arrayref msg');
+
+    @warnings = ();
+    %hash = sub { print "fenice" };
+    cmp_ok(scalar(@warnings),'==',1,'coderef count');
+    cmp_ok(substr($warnings[0],0,length($fail_odd)),'eq',$fail_odd,'coderef msg');
+
+    @warnings = ();
+    $_ = { 1..10 };
+    cmp_ok(scalar(@warnings),'==',0,'hashref assign');
+
+}
diff --git a/ext/Hash/Util/Makefile.PL b/ext/Hash/Util/Makefile.PL
index a328bfee85..7b7c166c75 100644
--- a/ext/Hash/Util/Makefile.PL
+++ b/ext/Hash/Util/Makefile.PL
@@ -8,6 +8,7 @@ WriteMakefile(
     MAN3PODS        => {},  # Pods will be built by installman.
     NAME            => "Hash::Util",
     DEFINE          => "-DPERL_EXT",
+    DIR             => ['FieldHash'],
 );
 
 package MY;
diff --git a/ext/Hash/Util/lib/Hash/Util.pm b/ext/Hash/Util/lib/Hash/Util.pm
index c62a8bf2b6..a4f143ea85 100644
--- a/ext/Hash/Util/lib/Hash/Util.pm
+++ b/ext/Hash/Util/lib/Hash/Util.pm
@@ -10,6 +10,8 @@ use Scalar::Util qw(reftype);
 require Exporter;
 our @ISA        = qw(Exporter);
 our @EXPORT_OK  = qw(
+                     fieldhash fieldhashes
+
                      all_keys
                      lock_keys unlock_keys
                      lock_value unlock_value
@@ -26,11 +28,21 @@ our @EXPORT_OK  = qw(
                      hash_seed hv_store
 
                     );
-our $VERSION    = 0.06;
+our $VERSION    = 0.07;
 require DynaLoader;
 local @ISA = qw(DynaLoader);
 bootstrap Hash::Util $VERSION;
 
+sub import {
+    my $class = shift;
+    if ( grep /fieldhash/, @_ ) {
+        require Hash::Util::FieldHash;
+        Hash::Util::FieldHash->import(':all'); # for re-export
+    }
+    unshift @_, $class;
+    goto &Exporter::import;
+}
+
 
 =head1 NAME
 
@@ -38,6 +50,20 @@ Hash::Util - A selection of general-utility hash subroutines
 
 =head1 SYNOPSIS
 
+  # Field hashes
+
+  use Hash::Util qw(fieldhash fieldhashes);
+
+  # Create a single field hash
+  fieldhash my %foo;
+
+  # Create three at once...
+  fieldhashes \ my(%foo, %bar, %baz);
+  # ...or any number
+  fieldhashes @hashrefs;
+
+  # Restricted hashes
+
   use Hash::Util qw(
                      hash_seed all_keys
                      lock_keys unlock_keys
@@ -79,6 +105,19 @@ don't really warrant a keyword.
 
 By default C<Hash::Util> does not export anything.
 
+=head2 Field hashes
+
+Field hashes are designed to maintain an association of a reference
+with a value. The association is independent of the bless status of
+the key, it is thread safe and garbage-collected.  These properties
+are desirable in the construction of inside-out classes.
+
+When used with keys that are plain scalars (not references), field
+hashes behave like normal hashes.
+
+Field hashes are defined in a separate module for which C<Hash::Util>
+is a front end.  For a detailed description see L<Hash::Util::FieldHash>.
+
 =head2 Restricted hashes
 
 5.8.0 introduces the ability to restrict a hash to a certain set of