# Copyright (C) 2001-2010, Parrot Foundation.

=head1 PDD 17: Polymorphic Containers

=head2 Abstract

This PDD describes the internal structure and behavior of the Polymorphic
Container (PMC) data type.

=head2 Description

PMCs implement all internal data types more complex than a simple integer,
float, or string, and also the data types of high-level languages.  Nothing
outside the core of Parrot (in fact, nothing outside the data type's vtable
routines) should infer anything about a PMC.

This does mean, though, that you need to either know what functions are
available and what they do, or have some method of finding out. Parrot defines
a standard set of functions that each PMC provides. More complex features are
constructed out of these fundamental building blocks.

=over 4

=item - PMCs contain both state and behavior

=item - PMCs can inherit from other PMCs

=item - PMCs can be composed of low-level roles

=item - High-level objects can subclass low-level PMCs


=back

=head2 Implementation

=head3 Internal structure

All PMCs have the form:

    struct PMC {
        Parrot_UInt flags;
        VTABLE *vtable;
        DPOINTER *data;
        PMC *_metadata;
    }

C<flags> holds a set of flags associated with the PMC; these are documented
in F<include/parrot/pobj.h>, and are generally only used within the Parrot
internals.

C<vtable> holds a pointer to the B<vtable> associated with the PMC. This
points to a set of functions, with interfaces described in L<Vtable Functions>
that implement the basic behaviour of the PMC (i.e. how it behaves under
addition, subtraction, cloning etc.)

C<data> holds a pointer to the core data associated with the PMC. This
may be null.

C<_metadata> holds internal PMC metadata (properties). See the setprop/getprop
ops in F<docs/ops/pmc.pod>.

C<_synchronize> is for access synchronization between shared PMCs.

PMCs are used to implement the basic data types of the high level languages
running on top of Parrot. For instance, a Perl 5 C<SV> will map onto one (or
more) types of PMC, while particular Python datatypes will map onto different
types of PMC.

=head3 Defining PMCs

PMCs are declared by the C<pmclass> keyword:

  pmclass <name> [ <modifier> ... ] {
  }

The modifiers specify core features, such as:

=over 4

=item abstract

The PMC cannot be instantiated. (By convention, abstract classes are given
lower-case names.)

=item no_init

Don't generate class initialization code (don't set up a vtable for the PMC).
Used with C<abstract>.

=item dynpmc

The PMC is a dynamic PMC, so generate special class initialization code
suitable for dynamic loading at runtime.

=item singleton

The PMC is a singleton, created in the constant PMC pool.

=item no_ro

Don't create a second read-only vtable for the PMC. (Used, for example,
by STM's C<share_ro()>.)

=item is_shared

The PMC is shared (across threads).

=item extends

Inherit from another PMC. Takes one argument, the name of the PMC to inherit
from.

=item does

Compose a role. Takes one argument, the name of the role to compose. [NOTE:
this modifier has been taken from the older feature C<does> which is now
called C<provides>.]

=item resolves

Resolve a conflicting vtable function, method, or attribute from a composed
role by using the same named vtable function, method, or attribute from the
current C<pmclass> or C<prole>.

=item provides

The PMC satisfies a particular low-level interface, which gives some
assurances on how and where a PMC can be used.

Roles composed with C<does> may also define C<provides> for one or more
interfaces. (They generally define at least a C<provides> corresponding
to their own name.)

=over 4

=item * C<array> is an aggregate PMC with numerically-keyed elements

=item * C<hash> is an aggregate PMC with string-keyed elements

=item * C<library> corresponds to a dynamic library

=item * C<ref> references another PMC

=item * C<string> behaves similarly to the base string type

=item * C<integer> behaves similarly to the base int type

=item * C<float> behaves similarly to the base number type

=item * C<boolean> does true/false only.

=item * C<scalar> (only used by the sample src/dynpmc/foo.pmc)

=item * C<event> can be used with event queue

=back

=item hll

Declare the PMC in an HLL namespace other than the default HLL 'parrot'.
Takes one argument, the name of the HLL.

=item maps

Map the current PMC to a core PMC type for code declared in a particular
HLL. May only be used together with the C<hll> modifier.

=back

=head4 Defining attributes

The attributes of a PMC (both public and private) live in a custom
struct for the PMC, stored in the C<data> member of the C<PMC> struct.
The standard way of declaring attributes is with C<ATTR> within the body
of a C<pmclass> or C<prole> declaration.

  ATTR <type> <name> [ :<modifier> ... ];

This declaration is used to generate the data struct for the PMC
(named "Parrot_<pmcname>_attributes").
The data struct incorporates any attributes declared
in a composed role or inherited class. Composed attributes are inserted at the
end of the generated struct. Inherited attributes are inserted at the top of
the generated struct, so the struct members shared by the parent and child are
in the same position, and code compiled for direct struct access to the parent
can also perform the same direct struct access on the child.

The declaration is also used to generate accessor macros, C<GET_ATTR_attrname>
and C<SET_ATTR_attrname>, to set up helper information for the C<inspect>
vtable function, and to provide attribute access through the default
C<get_attr> and C<set_attr> vtable functions.

It's also possible to define and store the PMC's data struct manually,
but the standard declaration syntax must be used in roles, in PMCs that
compose roles, and in PMCs that will be subclassed by high-level classes
(this means all core PMCs, and most non-core PMCs).

Low-level PMCs that use multiple inheritance (C<pmclass> declarations with
more than one C<extends> entry) have to be careful with the contents of their
structs. There's no problem if the two parents have identical data struct
members. But, if the two parents have different data struct members, any
vtable functions inherited from the parents will not be able to directly
access the parents' struct members or use the accessor macros. The solution is
to either override those vtable functions that directly access data struct
members in the child PMC, define the parents as roles instead of classes, or
declare the multiply inheriting child in PIR or an HLL.

=head4 Defining vtable functions

Vtable functions are defined as C functions within the body of the C<pmclass>
declaration, and are marked with C<VTABLE>.

  VTABLE STRING *get_string() {...}
  VTABLE void set_string_native(STRING *value) {...}

Within the body of vtable functions, several shortcuts are provided:

=over 4

=item INTERP

The current interpreter object.

=item SELF

The current invocant by dynamic type.

=item STATICSELF

The current invocant by static type.

=item SUPER

Calls the current method in the nearest superclass, using the dynamic
type of C<SELF>.

=item STATICSUPER

Calls the current method in the nearest superclass, using the static
type of C<SELF>.

=back

=head4 Methods

Methods are declared in the body of the C<pmclass> or C<prole>
declaration with C<METHOD>.

  METHOD inspect(STRING *what :optional, int got_what :opt_flag) {...}

=head3 PMCs and Namespaces

Like high-level classes, low-level PMCs are tied to a corresponding
namespace. By default this is a namespace with the same name as the PMC
in the 'parrot' HLL, but the C<hll> modifier in the C<pmclass>
declaration selects a different HLL.

Accessing a core PMC type from within an HLL other than 'parrot'
requires the same steps as accessing a class from another HLL, first
retrieving the namespace object, and then instantiating from that
namespace.

=begin PIR_FRAGMENT

  $P0 = get_root_namespace ['parrot'; 'Integer']
  $P1 = new $P0

=end PIR_FRAGMENT

HLLs can choose to provide direct access to Parrot's core PMC types by
aliasing them within the HLL namespace.

=head3 Inheritance

PMCs can inherit behavior and state from other PMCs. Inheritance is performed
in the C<pmclass> declaration using the C<extends> keyword.

  pmclass Foo extends Bar {
  }

=head3 Composition

Composition is another form of code reuse in PMCs. Unlike inheritance,
composed roles aren't complete stand-alone PMCs, they are just bundles of
behavior and state that can be used within the composing PMC. As such, roles
are never instantiated directly, and are never translated to C directly. They
have no core structs, though they define attributes to be added to the PMC
they are composed into.

When a PMC that uses a role is translated to C, the role provides vtable
functions, methods, and attributes that will be added to the generated C
code for that PMC. Composed vtable functions, methods, and attributes
are not permitted to have the same name as corresponding features
defined in the composing PMC. This is called a conflict, and must be
explicitly resolved in the composing PMC. When composed features have
the same name as inherited vtable functions, methods, or attributes, the
composed feature overrides the inherited feature, just as it would if
defined in the composing PMC.

Roles are defined using the C<prole> keyword.

  prole <name> <modifiers> {
  }

Roles can compose other roles, but they can't inherit.

Role attributes are defined using the same format as PMC attributes.

Core roles live in src/role and have a file extension of C<.pr>.

Roles are composed into a PMC with the C<does> modifier.

=head3 PMCs and high-level objects

High-level objects, as specified in PDD15, need to be able to inherit
from PMCs. Subclassing a low-level PMC from a high-level class makes an
entry in the high-level class's list of parents.

For PDD15 objects, there is a corresponding instance of the C<Class>
PMC. For a low-level PMC, however, the class definition is written in C
and compiled away. There needs to be something placed in the parents
list for a PDD15 class, that can provide access to the low-level PMC's
vtable and methods, and define the storage that the low-level PMC will
need within the high-level object. That something is the C<PMCProxy>
PMC. Like a PDD15 class, it is stored as the C<class> element in the
namespace associated with a PMC, provides introspection facilities and
can sit in an inheritance hierarchy.

The PMCProxy PMCs are only created when needed for subclassing a low-level
PMC, to avoid a large load of unused PMCProxy objects. When created, they are
cached in the class slot of the namespace corresponding to the low-level PMC,
so they are only created once.

Therefore, subclassing a PMC looks, at a PIR level, like subclassing a high
level class.

=begin PIR_FRAGMENT

  $P0 = get_class 'Hash'
  $P1 = newclass 'MyClass'
  addparent $P1, $P0     # The new class inherits from the Hash PMC

=end PIR_FRAGMENT

Or, more briefly:

=begin PIR_FRAGMENT

  $P1 = subclass 'Hash', 'MyClass'

=end PIR_FRAGMENT

PMCs store state in a very different way to PDD15 objects. When a method
inherited from a PMC is called on a PDD15 object, that method needs to
access the attributes of the inherited low-level PMC. Further, if
multiple PMCs are inherited from, they may each have attributes with the
same name, that need to be correctly "visible" to the PDD 15 object
according to the laws of inheritance. Users of Parrot at a PIR level
should not have to care about such issues.

To enable attributes from the low-level PMC to act as full inherited
attributes in the child class, the PMCProxy class will create a set of
PDD 15 attributes that correspond in type and name to the attributes
declared with C<ATTR> in the declaration body of the low-level PMC, as
if each had been added with C<add_attribute>. It will also override the
C<GET_ATTR_attrname> and C<SET_ATTR_attrname> functions to point to the
PDD 15 attributes (with automatic boxing and unboxing for the PMC
values) rather than to members of a C struct.

The PMCProxy will also scan the low-level PMC for methods declared with
C<METHOD> and insert them in the proxy class as if each had been
declared with C<add_method> (possibly with a shim to standardize calling
conventions, but hopefully the calling conventions are similar enough
between C-defined and PIR-defined methods not to need a shim). The
PMCProxy will maintain a link to the low-level PMC's vtable, and use it
for any vtable calls that aren't overridden by the proxy class itself.

As a result, a low-level PMC used as a parent of a PDD 15 class will
never be instantiated directly. It will only be used as a source for
attribute names and types, methods, and a vtable.

When a method is called on an object whose class has low-level PMC
parents, the call is made exactly as it would be for PDD 15 parents. The
invocant is always the PDD 15 object. Any method or vtable calls made
within the low-level PMC are dispatched on the PDD 15 object invocant.
This allows the PDD 15 object to intelligently handle method and vtable
overrides within multiple parents and itself.

If a low-level PMC expects to be overridden by high-level classes (which
means all the core low-level PMC types), it must respect the standard
interface.


=head2 Definitions

=head3 Vtable Functions

Vtables decouple the interface and implementation of various object functions.
The actual vtable structure contains pointers to functions that implement the
methods for that particular PMC.  All pointers must point to valid functions
with appropriate prototypes.

In C code, the first parameter to any vtable routine is the current
interpreter. The second parameter is the PMC itself.

The following list details each of the vtable functions, their prototypes, and
their behavior.

=head4 Core Vtable Functions

=over 4

=item init

  void init(INTERP, PMC *self)

Called when a PMC is first instantiated. It takes an unused PMC parameter and
turns it into a PMC of the appropriate class.

=item init_pmc

  void init_pmc(INTERP, PMC *self, PMC *initializer)

Alternative entry point called when a PMC is first instantiated.  Accepts a
PMC parameter used to initialize the given object.  Interpretation of the PMC
initializer is left open, each PMC is free to choose its own implementation. A
null value passed as the initializer parameter is allowed.

NOTE: It is strongly suggested that init_pmc(PMCNULL) be equivalent to
init(), though there will of necessity be exceptions.

=item init_init

  void init_int(INTERP, PMC *self, INTVAL initializer)

Alternative entry point called when a PMC is first instantiated.  Accepts an
integer parameter used to initialize the given object.  Interpretation of the
integer initializer is left open, each PMC is free to choose its own
implementation.  This is very useful for PMCs that require an integer size
parameter and is used as such in FixedPMCArray and related.

=item instantiate

  PMC* instantiate(INTERP, PMC *self, PMC *init)

Creates a new PMC object of the type of the class and calls init_pmc(),
passing in the initialization argument, I<init>, which is usually a
hash. This opcode is most often used with high-level classes, but
low-level PMCs may instantiate an object of their own type.

=item inspect

  PMC* inspect(INTERP, PMC *self)

Return a hash of all characteristics of the I<self> PMC available for
introspection.

  PMC* inspect_str(INTERP, PMC *self, STRING *what)

Return a PMC value for one characteristic of the I<self> PMC, selected
by string name. Returns PMCNULL if the characteristic doesn't exist.

=item morph

  void morph(INTERP, PMC *self, INTVAL type)

Turn the PMC into a PMC of type I<type>. If the morphing can't be done in any
reasonable way -- for instance if an integer is asked to turn into an Array --
then the PMC is first destroyed, then recreated as an empty PMC of the new
type.

This method is primarily used when the interpreter has need of coercing a PMC
to a particular type, and isn't meant as a general purpose casting tool.
Compilers should only emit valid morphing operations.

=item mark

  void mark(INTERP, PMC *self)

Called when the GC is tracing live PMCs. If this method is called then the
code must mark all strings and PMCs that it contains as live, otherwise they
may be collected.

This method is only called if the GC has detected that this PMC is both alive
and has a custom mark routine as indicated by the custom mark PMC flag.  (Most
normal PMCs don't need a custom mark routine.)

If a PMC has this flag set, then it is responsible for marking all buffers and
PMCs under its control as alive. If it does not, those PMCs or buffers may be
collected later. This method does I<not> have to call the C<mark> method on
any PMCs it marks--the GC system takes care of that. (So no need to recurse
into aggregate PMCs or anything of the sort).

This method may allocate no memory from Parrot, nor may it alter Parrot's
internal structures. It should have no side-effects from the C level either.
This routine may not throw an exception.

=item destroy

  void destroy(INTERP, PMC *self)

Called when the PMC is destroyed. This method is called by the GC when it
determines that a PMC is dead and that the PMC has marked itself as having a
destroy method (an active finalizer).

When this method finishes, the PMC will be marked as dead. As such you should
make sure that you don't leave any references to it in any Parrot structure
by the end of the method.

This method may not throw an exception. It will be ignored if it does.

=item clone

  PMC* clone(INTERP, PMC *self)

Return a clone of a PMC.

=item defined

  INTVAL defined(INTERP, PMC *self)

Return a true value if the PMC is defined, false otherwise.

=item get_class

  PMC* get_class(INTERP, PMC *self)

Return the class object for this PMC. For high-level objects, this is a
C<Class> object (or other high-level class object). For low-level PMCs,
this returns a C<PMCProxy> object.

=item get_namespace

  PMC* get_namespace(INTERP, PMC *self)

Return the namespace object for this PMC.

=item freeze

  void freeze(INTERP, PMC *self, visit_info* info)

Freeze the PMC to an archived string format (a bytecode string constant
that can be saved in a packfile).

=item thaw

  void thaw  (INTERP, PMC *self, visit_info* info)

Thaw a PMC from an archived string format (a bytecode string constant
that can be saved in a packfile).

=item thawfinish

  void thawfinish (INTERP, PMC *self, visit_info* info)

Called after the PMC has been thawed to perform any finalization steps.

=item visit

  void visit (INTERP, PMC *self, visit_info* info)

Used by C<freeze> and C<thaw> to visit the contents of the PMC.

=item share

  void share(INTERP, PMC *self)

  PMC* share_ro(INTERP, PMC *self)

Mark a PMC as shared and read-only.

[NOTE: these seem to be used interchangeably, should be distinguished or
merged.]

=back

=head4 Accessors

=over 4

=item getprop

  PMC* getprop(INTERP, PMC *self, STRING *key)

Return the value from the property hash of I<self> keyed by I<key>. The key
should not be null.

=item setprop

  void setprop(INTERP, PMC *self, STRING *key, PMC *value)

Set the value in the property hash of I<self> that is keyed by I<key> to the
value of I<value>. The key should not be null.

=item delprop

  void delprop(INTERP, PMC *self, STRING *key)

Delete the value from the property hash of I<self> keyed by I<key>. The key
should not be null.

=item getprops

  PMC* getprops(INTERP, PMC *self)

Return the entire property hash for I<self>.

=item type

  INTVAL type(INTERP, PMC *self)

Return the type of the PMC. Type is a unique number associated with the PMC
when the PMC's class is loaded. Negative numbers are considered
interpreter-specific, non-public types.

=item name

  STRING* name(INTERP, PMC *self)

Return the name of the class for the PMC.

=item get_integer

  INTVAL get_integer(INTERP, PMC *self)

Return the native integer value of the PMC.

=item get_number

  FLOATVAL get_number(INTERP, PMC *self)

Return the native floating-point value of the PMC.

=item get_string

  STRING* get_string(INTERP, PMC *self)

Return the native string value of the PMC. This may be in any encoding, chosen
by the PMC.

=item get_repr

  STRING* get_repr(INTERP, PMC *self)

Return a string representation of the PMC. [NOTE: redundant with
C<get_string>, candidate for deprecation.]

=item get_bool

  INTVAL get_bool(INTERP, PMC *self)

Return the true/false value of the PMC (the constant TRUE or the constant
FALSE). The definition of truth for a given PMC will depend on the type of the
PMC. For a scalar, it may be as simple as returning false when the PMC has a
value 0 or "", and returning true when the PMC has any other value.

=item get_pmc

  PMC* get_pmc(INTERP, PMC *self)

Return the PMC value for this PMC. This is useful in circumstances where the
thing being accessed may return something other than its own value. For
example, an array might return a reference to itself. Any PMC may return a
value different from the PMC that C<get_pmc> is being called on.

=item set_integer_native

  void set_integer_native(INTERP, PMC *self, INTVAL value)

Set the integer value of this PMC from a native integer value (integer
register/constant).

=item set_integer_same

  void set_integer_same(INTERP, PMC *self, PMC *value)

Set the value of this PMC from the integer value of another PMC. The value PMC
is guaranteed to be of the same type as the I<self> PMC, so optimizations may
be made.

=item set_number_native

  void set_number_native(INTERP, PMC *self, FLOATVAL value)

Set the value of this PMC from a native floating-point value (float
register/constant).

=item set_number_same

  void set_number_same(INTERP, PMC *self, PMC *value)

Set the value of this PMC from the floating-point value another PMC. The value
PMC is guaranteed to be of the same type as the I<self> PMC, so optimizations
may be made.

=item get_pointer

  void* get_pointer(INTERP, PMC *self)

Returns a pointer value for the PMC. Useful for PMCs that hold pointers to
arbitrary data. The details of the data (type, location etc.) depend on the
PMC.

=item set_bignum_int

  void set_bignum_int(INTERP, PMC *self, INTVAL value)

Morph the PMC to a BIGNUM PMC, and set the extended-precision value from a
native integer.

=item set_string_native

  void set_string_native(INTERP, PMC *self, STRING *value)

Set the value of this PMC from a native string value (string
register/constant).

=item assign_string_native

  void assign_string_native(INTERP, PMC *self, STRING *value)

Set the value of this PMC to a copied native string value (string
register/constant).

=item set_string_same

  void set_string_same(INTERP, PMC *self, PMC *value)

Set the value of this PMC from the string value of another PMC. The value PMC
is guaranteed to be of the same type as the I<self> PMC, so optimizations may
be made.

=item set_bool

  void set_bool(INTERP, PMC *self, INTVAL value)

Set the boolean state of the PMC to TRUE if the native integer value passed in
is TRUE, or FALSE if the value is FALSE. The definition of truth is left open
to the particular PMC. For a scalar, it may be as simple as setting false when
a 0 value is passed in, and seting true when any other value is passed in.

=item assign_pmc

  void assign_pmc(INTERP, PMC *self, PMC *value)

Set the value of the PMC in I<self> to the value of the PMC in I<value> by
copying the value.

=item set_pmc

  void set_pmc(INTERP, PMC *self, PMC *value)

Make the PMC in I<self> refer to the PMC passed as I<value>.

=item set_pointer

  void set_pointer(INTERP, PMC *self, void* value)

Set the pointer value of the PMC Useful for PMCs that hold pointers to
arbitrary data. The details of the data (type, location etc.) depend on the
PMC.

=back

=head4 Aggregate Vtable Functions

Many of the following functions have a *_keyed form, a *_keyed_int form, and a
*_keyed_str form. The keyed forms take a PMC *, INTVAL, or STRING * key as a
parameter. The PMC * parameter is null (PMCNULL) if there is no key for that
PMC; this means that that argument is unkeyed.

In some cases, the caller must provide a non-null key.  Those cases are
explicitly stated below.  In the other cases, you may have to implement the
keyed vtable functions and check for a null I<self> key even if you are
implementing a non-aggregate type.  If the I<self> key is non-null and the PMC
class is a non-aggregate type, the _keyed_* methods should throw an exception.

If you do not implement the *_keyed_int and *_keyed_str functions, the default
will convert the INTVAL or STRING * into a key PMC * and call the
corresponding *_keyed functions.

=over 4

=item elements

  INTVAL elements(INTERP, PMC *self)

Return the number of elements in the PMC.

=item get_integer_keyed

  INTVAL get_integer_keyed(INTERP, PMC *self, PMC *key)
  INTVAL get_integer_keyed_int(INTERP, PMC *self, INTVAL key)
  INTVAL get_integer_keyed_str(INTERP, PMC *self, STRING *key)

Return the integer value for the element indexed by a PMC, integer, or string
key. The key is guaranteed not to be null for this function.

=item get_number_keyed

  FLOATVAL get_number_keyed(INTERP, PMC *self, PMC *key)
  FLOATVAL get_number_keyed_int(INTERP, PMC *self, INTVAL key)
  FLOATVAL get_number_keyed_str(INTERP, PMC *self, STRING *key)

Return the native floating-point value for the element indexed by a PMC,
integer, or string key. The key is guaranteed not to be null for this
function.

=item get_string_keyed

  STRING* get_string_keyed(INTERP, PMC *self, PMC *key)
  STRING* get_string_keyed_int(INTERP, PMC *self, INTVAL key)
  STRING* get_string_keyed_str(INTERP, PMC *self, STRING *key)

Return the string value for the element indexed by a PMC, integer, or string
key. The key is guaranteed not to be null for this function.

=item get_pmc_keyed

  PMC* get_pmc_keyed(INTERP, PMC *self, PMC *key)
  PMC* get_pmc_keyed_int(INTERP, PMC *self, INTVAL key)
  PMC* get_pmc_keyed_str(INTERP, PMC *self, STRING *key)

Return the PMC value for the element indexed by a PMC, integer, or
string key. The key is guaranteed not to be null for this function.

=item get_pointer_keyed

  void* get_pointer_keyed(INTERP, PMC *self, PMC *key)
  void* get_pointer_keyed_int(INTERP, PMC *self, INTVAL key)
  void* get_pointer_keyed_str(INTERP, PMC *self, STRING *key)

Return the pointer value for the element indexed by a PMC, integer, or
string key. The details of the data (type, location etc.) depend on the
PMC.

=item set_integer_keyed

  void set_integer_keyed(INTERP, PMC *self, PMC *key, INTVAL value)
  void set_integer_keyed_int(INTERP, PMC *self, INTVAL key, INTVAL value)
  void set_integer_keyed_str(INTERP, PMC *self, STRING *key, INTVAL value)

Set the integer value of the element indexed by a PMC, integer, or
string key. The key is guaranteed not to be null for this function.

=item set_number_keyed

  void set_number_keyed(INTERP, PMC *self, PMC *key, FLOATVAL value)
  void set_number_keyed_int(INTERP, PMC *self, INTVAL key, FLOATVAL value)
  void set_number_keyed_str(INTERP, PMC *self, STRING *key, FLOATVAL value)

Set the floating-point value of the element indexed by a PMC, integer,
or string key. The key is guaranteed not to be null for this function.

=item set_string_keyed

  void set_string_keyed(INTERP, PMC *self, PMC *key, STRING *value)
  void set_string_keyed_int(INTERP, PMC *self, INTVAL key, STRING *value)
  void set_string_keyed_str(INTERP, PMC *self, STRING *key, STRING *value)

Set the string value of the element indexed by a PMC, integer, or string
key. The key is guaranteed not to be null for this function.

=item set_pmc_keyed

  void set_pmc_keyed(INTERP, PMC *self, PMC *key, PMC *value)
  void set_pmc_keyed_int(INTERP, PMC *self, INTVAL key, PMC *value)
  void set_pmc_keyed_str(INTERP, PMC *self, STRING *key, PMC *value)

Set the PMC value of the element indexed by a PMC, integer, or string key by
copying the value of another PMC.

=item set_pointer_keyed

  void set_pointer_keyed(INTERP, PMC *self, PMC *key, void* value)
  void set_pointer_keyed_int(INTERP, PMC *self, INTVAL key, void* value)
  void set_pointer_keyed_str(INTERP, PMC *self, STRING *key, void* value)

Set the pointer value of the element indexed by a PMC, integer, or string key.

=item pop_integer

  INTVAL pop_integer(INTERP, PMC *self)

Return the integer value of the last item on the list, removing that item.

=item pop_float

  FLOATVAL pop_float(INTERP, PMC *self)

Return the floating-point value of the last item on the list, removing that
item.

=item pop_string

  STRING* pop_string(INTERP, PMC *self)

Return the string value of the last item on the list, removing that item.

=item pop_pmc

  PMC* pop_pmc(INTERP, PMC *self)

Return the PMC value of the last item on the list, removing that item.

=item push_integer

  void push_integer(INTERP, PMC *self, INTVAL value)

Add the passed in integer value to the end of the list.

=item push_float

  void push_float(INTERP, PMC *self, FLOATVAL value)

Add the passed in floating-point number to the end of the list.

=item push_string

  void push_string(INTERP, PMC *self, STRING *value)

Add the passed in string to the end of the list.

=item push_pmc

  void push_pmc(INTERP, PMC *self, PMC *value)

Add the passed in PMC to the end of the list.

=item shift_integer

  INTVAL shift_integer(INTERP, PMC *self)

Return the integer value of the first item on the list, removing that item.

=item shift_float

  FLOATVAL shift_float(INTERP, PMC *self)

Return the floating-point value of the first item on the list, removing that
item.

=item shift_string

  STRING* shift_string(INTERP, PMC *self)

Return the string value of the first item on the list, removing that item.

=item shift_pmc

  PMC* shift_pmc(INTERP, PMC *self)

Return the PMC value of the first item on the list, removing that item.

=item unshift_integer

  void unshift_integer(INTERP, PMC *self, INTVAL value)

Add the passed in integer value to the beginning of the list.

=item unshift_float

  void unshift_float(INTERP, PMC *self, FLOATVAL value)

Add the passed in floating-point number to the beginning of the list.

=item unshift_string

  void unshift_string(INTERP, PMC *self, STRING *value)

Add the passed in string to the beginning of the list.

=item unshift_pmc

  void unshift_pmc(INTERP, PMC *self, PMC *value)

Add the passed in PMC to the beginning of the list.

=item splice

  void splice(INTERP, PMC *self, PMC *value, INTVAL offset, INTVAL count)

Replace some number of PMCs (specified by the integer I<count>) at
offset I<offset> from the beginning of I<self> with the PMCs in the
aggregate I<value>.

=item exists_keyed

  INTVAL exists_keyed(INTERP, PMC *self, PMC *key)
  INTVAL exists_keyed_int(INTERP, PMC *self, INTVAL key)
  INTVAL exists_keyed_str(INTERP, PMC *self, STRING *key)

Check if the element indexed by a PMC, integer, or string key exists.

=item defined_keyed

  INTVAL defined_keyed(INTERP, PMC *self, PMC *key)
  INTVAL defined_keyed_int(INTERP, PMC *self, INTVAL key)
  INTVAL defined_keyed_str(INTERP, PMC *self, STRING *key)

Check if the element indexed by a PMC, integer, or string key is defined.

=item delete_keyed

  void delete_keyed(INTERP, PMC *self, PMC *key)
  void delete_keyed_int(INTERP, PMC *self, INTVAL key)
  void delete_keyed_str(INTERP, PMC *self, STRING *key)

Delete the element indexed by a PMC, integer, or string key.

=back

=head4 Math Vtable Functions

=over 4

=item add

  void add(INTERP, PMC *self, PMC *value, PMC *dest)
  void add_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void add_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_add(INTERP, PMC *self, PMC *value)
  void i_add_int(INTERP, PMC *self, INTVAL value)
  void i_add_float(INTERP, PMC *self, FLOATVAL value)

Add the value of I<self> to the value of a PMC, native integer, or native
floating-point number and store the result in a PMC I<dest>. Note that I<dest>
may be the same PMC as I<self>; in that case optimizations may be made.
The C<i_> variants perform an inplace operation, modifying the value of
I<self>.

=item subtract

  PMC* subtract(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* subtract_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  PMC* subtract_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_subtract(INTERP, PMC *self, PMC *value)
  void i_subtract_int(INTERP, PMC *self, INTVAL value)
  void i_subtract_float(INTERP, PMC *self, FLOATVAL value)

Subtract the value of a PMC, native integer, or native floating-point number
from a PMC and store the result in I<dest>. If I<dest> is null create a result
PMC of an appropriate type.  Note that I<dest> may be the same PMC as I<self>;
in that case optimizations may be made. The C<i_> variants perform an
inplace operation, modifying the value of I<self>.

=item increment

  void increment(INTERP, PMC *self)

Increment the value of a PMC by 1.

=item decrement

  void decrement(INTERP, PMC *self)

Decrement the value of a PMC by 1.

=item multiply

  void multiply(INTERP, PMC *self, PMC *value, PMC *dest)
  void multiply_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void multiply_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_multiply(INTERP, PMC *self, PMC *value)
  void i_multiply_int(INTERP, PMC *self, INTVAL value)
  void i_multiply_float(INTERP, PMC *self, FLOATVAL value)

Multiply a PMC, native integer, or floating-point value by the value of the
PMC I<self> and store the result in the I<dest> PMC. Note that I<dest> may be
the same PMC as I<self>; in that case optimizations may be made. The C<i_>
variants perform an inplace operation, modifying the value of I<self>.

=item divide

  void divide(INTERP, PMC *self, PMC *value, PMC *dest)
  void divide_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void divide_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_divide(INTERP, PMC *self, PMC *value)
  void i_divide_int(INTERP, PMC *self, INTVAL value)
  void i_divide_float(INTERP, PMC *self, FLOATVAL value)

Divide the value of the I<self> PMC by a PMC, native integer, or native
floating-point number and store the result in I<dest>.  Note that I<dest> may
be the same PMC as I<self>; in that case optimizations may be made. The
C<i_> variants perform an inplace operation, modifying the value of
I<self>.

=item floor_divide

  PMC* floor_divide(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* floor_divide_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  PMC* floor_divide_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_floor_divide(INTERP, PMC *self, PMC *value)
  void i_floor_divide_int(INTERP, PMC *self, INTVAL value)
  void i_floor_divide_float(INTERP, PMC *self, FLOATVAL value)

Divide the PMC's value number by I<value> and return the result in
I<dest>. The result is the C<floor()> of the division i.e. the next
whole integer towards -inf. If the denominator is zero, a 'Divide by
zero' exception is thrown. The C<i_> variants perform an inplace
operation, modifying the value of I<self>.

=item modulus

  void modulus(INTERP, PMC *self, PMC *value, PMC *dest)
  void modulus_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void modulus_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_modulus(INTERP, PMC *self, PMC *value)
  void i_modulus_int(INTERP, PMC *self, INTVAL value)
  void i_modulus_float(INTERP, PMC *self, FLOATVAL value)

Divide the value of the I<self> PMC by the value of a PMC, native integer, or
native floating-point number and store the remainder in I<dest>.  Note that
I<dest> may be the same PMC as I<self>; in that case optimizations may be
made.  The C<i_> variants perform an inplace operation, modifying the value of
I<self>.

=item cmodulus

  void cmodulus(INTERP, PMC *self, PMC *value, PMC *dest)
  void cmodulus_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void cmodulus_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_cmodulus(INTERP, PMC *self, PMC *value)
  void i_cmodulus_int(INTERP, PMC *self, INTVAL value)
  void i_cmodulus_float(INTERP, PMC *self, FLOATVAL value)

Divide the value of the I<self> PMC by the value of a PMC, native integer, or
native floating-point number and store the remainder in I<dest>.  Note that
I<dest> may be the same PMC as I<self>; in that case optimizations may be
made.  The C<i_> variants perform an inplace operation, modifying the value of
I<self>.

Note that C<modulus> uses Knuth's "corrected mod" algorithm, as implemented in
F<src/utils.c>, while C<cmodulus> uses the C-style fmod function.

=item pow

  PMC* pow(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* pow_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  PMC* pow_float(INTERP, PMC *self, FLOATVAL value, PMC *dest)

  void i_pow(INTERP, PMC *self, PMC *value)
  void i_pow_int(INTERP, PMC *self, INTVAL value)
  void i_pow_float(INTERP, PMC *self, FLOATVAL value)

Return the value of I<self> raised to the power of I<value>. The C<i_>
variants perform an inplace operation, modifying the value of I<self>.

=item absolute

  PMC* absolute(INTERP, PMC *self, PMC *dest)
  void i_absolute(INTERP, PMC *self)

Return the absolute value of I<self>. The C<i_> variant performs an
inplace operation, modifying the value of I<self>.

=item neg

  void neg(INTERP, PMC *self, PMC *dest)
  void i_neg(INTERP, PMC *self)

Negate the sign of I<self> and store the result in I<dest>. Note that
I<self> and I<dest> may refer to the same PMC, in which case
optimizations may be made. The C<i_> variant performs an inplace
operation, modifying the value of I<self>.

=back

=head4 Logical Vtable Functions

=over 4

=item bitwise_or

  void bitwise_or(INTERP, PMC *self, PMC *value, PMC *dest)
  void bitwise_or_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_or(INTERP, PMC *self, PMC *value)
  void i_bitwise_or_int(INTERP, PMC *self, INTVAL value)

Calculate the bitwise-OR of the value of the I<self> PMC and the value of a
PMC or native integer and store the result in I<dest>. Note that I<dest> may
be the same PMC as I<self>; in that case optimizations may be made.
[Question: what happens when the I<self> and I<value> PMCs aren't integers?]

The C<i_> variants perform an inplace operation and store the result in
I<self>.

=item bitwise_and

  PMC* bitwise_and(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_and_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_and(INTERP, PMC *self, PMC *value)
  void i_bitwise_and_int(INTERP, PMC *self, INTVAL value)

Return the result of a bitwise AND on the passed in I<value> and the I<self>
PMC. The C<i_> variants perform an inplace operation and store the result in
I<self>.

=item bitwise_xor

  PMC* bitwise_xor(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_xor_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_xor(INTERP, PMC *self, PMC *value)
  void i_bitwise_xor_int(INTERP, PMC *self, INTVAL value)

Return the result of a bitwise XOR on the passed in I<value> and the I<self>
PMC. The C<i_> variants perform an inplace operation and store the result in
I<self>.

=item bitwise_ors

  PMC* bitwise_ors(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_ors_str(INTERP, PMC *self, STRING *value, PMC *dest)
  void i_bitwise_ors(INTERP, PMC *self, PMC *value)
  void i_bitwise_ors_str(INTERP, PMC *self, STRING *value)

Return the result of a bitwise OR over an entire string on the passed in
I<value> and the I<self> PMC. The C<i_> variants perform an inplace operation
and store the result in I<self>.

=item bitwise_ands

  PMC* bitwise_ands(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_ands_str(INTERP, PMC *self, STRING *value, PMC *dest)
  void i_bitwise_ands(INTERP, PMC *self, PMC *value)
  void i_bitwise_ands_str(INTERP, PMC *self, STRING *value)

Return the result of a bitwise AND over an entire string on the passed in
I<value> and the I<self> PMC. The C<i_> variants perform an inplace operation
and store the result in I<self>.

=item bitwise_xors

  PMC* bitwise_xors(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_xors_str(INTERP, PMC *self, STRING *value, PMC *dest)
  void i_bitwise_xors(INTERP, PMC *self, PMC *value)
  void i_bitwise_xors_str(INTERP, PMC *self, STRING *value)

Return the result of a bitwise XOR over an entire string on the passed in
I<value> and the I<self> PMC. The C<i_> variants perform an inplace operation
and store the result in I<self>.

=item bitwise_not

  PMC* bitwise_not(INTERP, PMC *self, PMC *dest)
  void i_bitwise_not(INTERP, PMC *self)

Returns the bitwise negation of the I<self> PMC. The C<i_> variant performs an
inplace operation, storing the result in I<self>.

=item bitwise_nots

  PMC* bitwise_nots(INTERP, PMC *self, PMC *dest)
  void i_bitwise_nots(INTERP, PMC *self)

Returns the bitwise negation of the string I<self> PMC. The C<i_> variant
performs an inplace operation, storing the result in I<self>.

=item bitwise_shl

  PMC* bitwise_shl(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_shl_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_shl(INTERP, PMC *self, PMC *value)
  void i_bitwise_shl_int(INTERP, PMC *self, INTVAL value)

Return the value of the I<self> PMC bitwise shifted left by the amount
specified in I<value>, shifting in zeroes on the right (arithmetic/logical
bitwise shift). A negative I<value> shifts right. The C<i_> variants perform
an inplace operation, storing the result in I<self>.

The result may be promoted to a C<BigInt>.

=item bitwise_shr

  PMC* bitwise_shr(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_shr_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_shr(INTERP, PMC *self, PMC *value)
  void i_bitwise_shr_int(INTERP, PMC *self, INTVAL value)

Return the value of the I<self> PMC bitwise shifted right by the amount
specified in I<value>, shifting in copies of the sign bit on the left
(arithmetic bitwise shift). A negative I<value> shifts left. The C<i_>
variants perform an inplace operation, storing the result in I<self>.

The result may be promoted to a C<BigInt> (when I<value> is negative).

=item bitwise_lsr

  PMC* bitwise_lsr(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* bitwise_lsr_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_bitwise_lsr(INTERP, PMC *self, PMC *value)
  void i_bitwise_lsr_int(INTERP, PMC *self, INTVAL value)

Return the value of the I<self> PMC bitwise shifted right by the amount
specified in I<value>, shifting in zeroes on the left (logical bitwise shift).
A negative I<value> shifts left. The C<i_> variants perform an inplace
operation, storing the result in I<self>.

=item is_equal

  INTVAL is_equal(INTERP, PMC *self, PMC *value)
  INTVAL is_equal_num(INTERP, PMC *self, PMC *value)
  INTVAL is_equal_string(INTERP, PMC *self, PMC *value)

Return an integer value (1/0) indicating if the I<self> PMC is equal to
the I<value> PMC. The C<_num> version tests for numeric equality, while
the C<_string> version tests for string equality.

=item is_same

  INTVAL is_same(INTERP, PMC *self, PMC *value)

Return an integer value (1/0) indicating if I<self> is the same as
I<value> (with "sameness" determined by each PMC).

=item cmp

  INTVAL cmp(INTERP, PMC *self, PMC *value)
  INTVAL cmp_num(INTERP, PMC *self, PMC *value)
  INTVAL cmp_string(INTERP, PMC *self, PMC *value)

Returns the integer result of comparing the values of I<self> and
I<value> (0 for equal, 1 if I<self> is greater, -1 if I<value> is
greater). The C<_num> version performs a numeric comparison, while the
C<_string> version performs a string comparison.

=back

=head4 String Vtable Functions

=over 4

=item concatenate

  PMC* concatenate(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* concatenate_str(INTERP, PMC *self, STRING *value, PMC *dest)
  void i_concatenate(INTERP, PMC *self, PMC *value)
  void i_concatenate_str(INTERP, PMC *self, STRING *value)

Concatenate I<self> with a PMC or string value and return the result.
The C<i_> variant performs an inplace concatenation, modifying the value
of I<self>.

=item repeat

  PMC* repeat(INTERP, PMC *self, PMC *value, PMC *dest)
  PMC* repeat_int(INTERP, PMC *self, INTVAL value, PMC *dest)
  void i_repeat(INTERP, PMC *self, PMC *value)
  void i_repeat_int(INTERP, PMC *self, INTVAL value)

Return the result of repeating the value in I<self> the number of times
indicated in I<value>. The C<i_> variants perform an inplace operation,
modifying the value of I<self>.

=item substr

  STRING* substr(INTERP, PMC *self, INTVAL offset, INTVAL length)

Extracts the string starting at I<offset> with size I<length> and return
it as a PMC in I<dest> or as a string return value.

=back

=head4 Code Vtable Functions

=over 4

=item invoke

  opcode_t* invoke(INTERP, PMC *self, void* next)

Invoke the code object I<self>.

=back

=head4 Class/Object Vtable Functions

=over 4

=item can

  INTVAL can(INTERP, PMC *self, STRING *method)

Return a true value if the PMC has a method named I<method>, return 0
otherwise.

=item isa

  INTVAL isa(INTERP, PMC *self, STRING *classname)

Return a true value if the PMC inherits from  the class named
I<classname>, return 0 otherwise.

=item does

  INTVAL does(INTERP, PMC *self, STRING *role)

Return a true value if the PMC C<does> (composes) or C<performs>
(satisfies the interface of) the role named I<role>, return 0 otherwise.

=item get_attr

  PMC* get_attr_str(INTERP, PMC *self, STRING *idx)

Retrieve an attribute value from the PMC (instance object).

=item set_attr

  void set_attr_str(INTERP, PMC *self, STRING *idx, PMC *value)

Store an attribute value in the PMC (instance object).

=item add_parent

  void add_parent(INTERP, PMC *self, PMC *parent)

Add a parent to the PMC (class object), establishing an inheritance relation.

=item remove_parent

  void remove_parent(INTERP, PMC *self, PMC *parent)

Remove a parent from a PMC (class object).

Not all object metamodels will support removing parents.

=item add_role

  void add_role(INTERP, PMC *self, PMC *role)

Add a role to the PMC (class object), establishing a composition relation.

=item remove_role

  void remove_role(INTERP, PMC *self, PMC *role)

Remove a role from the PMC (class object).

Not all object metamodels will support removing roles.

=item add_attribute

  void add_attribute(INTERP, PMC *self, STRING *name, PMC *type)

Add an attribute to the PMC (class object).

=item remove_attribute

  void remove_attribute(INTERP, PMC *self, STRING *name)

Remove an attribute from the PMC (class object).

Not all object metamodels will support removing attributes.

=item add_method

  void add_method(INTERP, PMC *self, STRING *method_name, PMC *sub_pmc)

Add a method to the PMC (class object).

=item remove_method

  void remove_method(INTERP, PMC *self, STRING *method_name)

Remove a method from the PMC (class object).

Not all object metamodels will support removing methods.

=item add_vtable_override

  void add_vtable_override(INTERP, PMC *self, STRING *vtable_name,
                           PMC *sub_pmc)

Add a vtable override to the PMC (class object).

  void remove_vtable_override(INTERP, PMC *self, STRING *vtable_name)

Remove a vtable override from the PMC (class object).

=item find_method

  PMC* find_method(INTERP, PMC *self, STRING *method_name)

Return a subroutine PMC for the passed method name. This subroutine PMC may be
cached, so the method I<must> return an equivalent sub PMC each time, or be
capable of dealing with the returned sub PMCs being reused. [Why should it be
cached? Can you turn off caching? What if you want to override find_method to
generate methods on the fly?]

=back



=head3 Core PMCs

Parrot has a number of core PMC types that all programs can guarantee will be
available to them. (With the possible exception of Parrot programs executing
on an embedded device or other restricted environment)

=head4 Scalar types

=over 4

=item Undef

This is the generic no-value type. It has a numeric value of zero, a string
value of empty string, and a boolean value of false. It will, on assignment,
turn itself into a PMC of the source type, or if assigned a basic type will
turn itself into one of the wrapper PMC types (detailed below) for the basic
types.

=item Integer

The PMC wrapper for Parrot's low-level integer type. Always an integer, with
other types auto-converted to an integer when stored into this PMC. The range
and behaviour of the Integer PMC is identical to the platform low-level
integer.

The boolean value for an Integer is false if zero, otherwise true.

Floating point, string, and bignum values assigned to an Integer PMC round to
the nearest integer. Floats, or strings which resolve to numbers, cap at the
platform maximum or minimum integer value.

Integer PMCs take on a value of 1 if a boolean true is assigned, and a value
of 0 if a boolean false is assigned.

If an out-of-range value is assigned to an Integer PMC, the PMC will throw an
exception if exact math is enabled.

=item Float

The PMC wrapper for Parrot's low-level floating-point type. Always a float,
with other types autoconverted to a float when stored into this PMC.

The boolean value for a Float is false if exactly zero, otherwise true.

When converted to an integer, floats round to the closest integer, capping at
the platform maximum or minimum integer value.

When converting to a string, floats use the platform default snprintf format.

=item String

The PMC wrapper for Parrot's low-level string type. Always a simple string,
with other types autoconverted to a string when stored into this PMC.

The boolean value for a String is false if empty or the string '0' (a one
character string holding a zero) otherwise true. This PMC autoconverts to an
integer or float when its integer or float value is fetched.

=item Boolean

A true/false value. Returns 0 for false, 1 for true when fetched as an
integer or float, empty string for false and '1' for true when fetched
as a string.

=item BigInt

An arbitrary precision integer.

=item BigNum

The PMC wrapper for Parrot's low-level BigNum type.
{{ NOTE: this type doesn't seem to exist. }}

=item Complex

A complex number, consisting of a real part and an imaginary part.
{{ NOTE: is this a complete and useful implementation of complex
numbers? }}

=item Class

The PMC for Parrot's class.

=item Object

The PMC for Parrot's base object type.

=item Ref

The PMC that represents a reference to another PMC. Delegates all functions to
the referred-to PMC.

=item AggregateElementRef

This PMC represents a reference to an element contained in an aggregate PMC
type, such as an array or hash. It is initialized with the key being
referenced and the aggregate PMC containing that key.

Note that assigning to the reference PMC will be equivalent to a keyed set on
the referenced aggregate PMC - that is, it modifies the element rather than
doing a v-table call on the element itself. It is important to be aware of
this when assigning a PMC through this reference; it is not the same behaviour
as the Ref PMC.

=item WeakRegisterRef

This PMC represents a weak reference to a register. Should the reference live
beyond the context containing the register that it references, any attempt to
use the reference will throw an exception.

A weak register reference can only be created by the C<register_ref> opcode.
Any assignment to the register will behave like a set instruction. That is,
when assigning a PMC using a WeakRegisterRef PMC, the register will be updated
to reference that PMC rather than calling the assign v-table call on the PMC
in that register. This is not the same behaviour as the Ref PMC.

=item Exception

The base class for all exceptions. Currently based on
C<ResizablePMCArray>, but that's likely to change.

=back

=head4 Array types

Note that for the following types you can set the size of the array by using
the VTABLE_set_integer_native() function. Assigning an integer to the array
as a whole sets the array to that size.

Size-changing operations (such as push, pop, shift, unshift, and splice)
on statically-sized arrays will throw an exception.

ResizablePMCArray returns Undef for unset elements (so does the new
object model, because it uses ResizablePMCArray for storage), but Hash
returns PMCNULL. Standardize all core aggregate PMC types on the
singleton PMCNULL.

=over 4

=item Array

The base class for all array types (a statically sized array for any
arbitrary type). New array types can be derived from the base Array.
In user code it is recommended to use one of the specific array types
below, rather than the base type.

=item FixedBooleanArray

A statically sized array which holds only Boolean values.

=item ResizableBooleanArray

A dynamically sized array which holds only Boolean values.

=item FixedIntegerArray

A statically sized array which holds only Integer values.

=item ResizableIntegerArray

A dynamically sized array which holds only Integer values.

=item FixedFloatArray

A statically sized array which holds only Float values.

=item ResizableFloatArray

A dynamically sized array which holds only Float values.

=item FixedPMCArray

A statically sized array which holds only PMC values.

=item ResizablePMCArray

A dynamically sized array which holds only PMC values.

=item FixedStringArray

A statically sized array which holds only String values.

=item ResizableStringArray

A dynamically sized array which holds only String values.

=back

=head4 Hash types

=over 4

=item Hash

A container with key-value semantics. The values are PMCs.

=item Env

Env is a singleton PMC class, that is there is only one Env PMC in any
interpreter. This PMC gives access to the process' environment
variables--reading from it returns the value of the named process environment
variable, while writing to it sets the value of a process environment
variable.  For example, to retrieve the current value of TERM (the terminal
type on most Unix systems):

   new P1, 'Env'
   set S1, P1['TERM']

Note that an embedding system may override this behavior.

=item NameSpace

Stores one level of a namespace. Every slot in a namespace contains
either another namespace (the next level down), or a variable or
subroutine/method.

=item OrderedHash

A hash that also preserves the order of elements, providing the
interface of both a hash and an array.

=item AddrRegistry

Simulates reference counting for dead-object detection and garbage
collection.

=back

=head4 Subroutine types

=over 4

=item Sub

A fundamental subroutine object, and base class for other subroutine
PMC types.

=item Closure

A closure: subroutine object plus captured lexical scope.

=item Continuation

A continuation: a subroutine object that captures the interpreter's
context at the point where the continuation was constructed.

=item Coroutine

A coroutine: a continuation object that can stop part way through
execution and restart at the point where it left off the next time it's
called.

=item Eval

An extension of C<Sub> that provides dynamic code evaluation and
execution.

=item ExceptionHandler

A code object for handling exceptions.

=item MultiSub

A container for multiply dispatched subroutines.

=item NCI

A native call interface wrapper around a C function.

=item Bound_NCI

An internal NCI method call bound to a particular call instance.

=item Compiler

A subroutine implementing a language compiler. (Derived from NCI.)

=back

=head2 References

F<docs/pmc2c.pod>

=cut

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