Provided by: perl-doc_5.38.2-3.2ubuntu0.1_all 
NAME
perlclassguts - Internals of how "feature 'class'" and class syntax works
DESCRIPTION
This document provides in-depth information about the way in which the perl interpreter implements the
"feature 'class'" syntax and overall behaviour. It is not intended as an end-user guide on how to use
the feature. For that, see perlclass.
The reader is assumed to be generally familiar with the perl interpreter internals overall. For a more
general overview of these details, see also perlguts.
DATA STORAGE
Classes
A class is fundamentally a package, and exists in the symbol table as an HV with an aux structure in
exactly the same way as a non-class package. It is distinguished from a non-class package by the fact
that the HvSTASH_IS_CLASS() macro will return true on it.
Extra information relating to it being a class is stored in the "struct xpvhv_aux" structure attached to
the stash, in the following fields:
HV *xhv_class_superclass;
CV *xhv_class_initfields_cv;
AV *xhv_class_adjust_blocks;
PADNAMELIST *xhv_class_fields;
PADOFFSET xhv_class_next_fieldix;
HV *xhv_class_param_map;
• "xhv_class_superclass" will be "NULL" for a class with no superclass. It will point directly to the
stash of the parent class if one has been set with the :isa() class attribute.
• "xhv_class_initfields_cv" will contain a "CV *" pointing to a function to be invoked as part of the
constructor of this class or any subclass thereof. This CV is responsible for initializing all the
fields defined by this class for a new instance. This CV will be an anonymous real function - i.e.
while it has no name and no GV, it is not a protosub and may be directly invoked.
• "xhv_class_adjust_blocks" may point to an AV containing CV pointers to each of the "ADJUST" blocks
defined on the class. If the class has a superclass, this array will additionally contain duplicate
pointers of the CVs of its parent class. The AV is created lazily the first time an element is pushed
to it; it is valid for there not to be one, and this pointer will be "NULL" in that case.
The CVs are stored directly, not via RVs. Each CV will be an anonymous real function.
• "xhv_class_fields" will point to a "PADNAMELIST" containing "PADNAME"s, each being one defined field
of the class. They are stored in order of declaration. Note however, that the index into this array
will not necessarily be equal to the "fieldix" of each field, because in the case of a subclass, the
array will begin at zero but the index of the first field in it will be non-zero if its parent class
contains any fields at all.
For more information on how individual fields are represented, see "Fields".
• "xhv_class_next_fieldix" gives the field index that will be assigned to the next field to be added to
the class. It is only useful at compile-time.
• "xhv_class_param_map" may point to an HV which maps field ":param" attribute names to the field index
of the field with that name. This mapping is copied from parent classes; each class will contain the
sum total of all its parents in addition to its own.
Fields
A field is still fundamentally a lexical variable declared in a scope, and exists in the "PADNAMELIST" of
its corresponding CV. Methods and other method-like CVs can still capture them exactly as they can with
regular lexicals. A field is distinguished from other kinds of pad entry in that the PadnameIsFIELD()
macro will return true on it.
Extra information relating to it being a field is stored in an additional structure accessible via the
PadnameFIELDINFO() macro on the padname. This structure has the following fields:
PADOFFSET fieldix;
HV *fieldstash;
OP *defop;
SV *paramname;
bool def_if_undef;
bool def_if_false;
• "fieldix" stores the "field index" of the field; that is, the index into the instance field array
where this field's value will be stored. Note that the first index in the array is not specially
reserved. The first field in a class will start from field index 0.
• "fieldstash" stores a pointer to the stash of the class that defined this field. This is necessary in
case there are multiple classes defined within the same scope; it is used to disambiguate the fields
of each.
{
class C1; field $x;
class C2; field $x;
}
• "defop" may store a pointer to a defaulting expression optree for this field. Defaulting expressions
are optional; this field may be "NULL".
• "paramname" may point to a regular string SV containing the ":param" name attribute given to the
field. If none, it will be "NULL".
• One of "def_if_undef" and "def_if_false" will be true if the defaulting expression was set using the
"//=" or "||=" operators respectively.
Methods
A method is still fundamentally a CV, and has the same basic representation as one. It has an optree and
a pad, and is stored via a GV in the stash of its containing package. It is distinguished from a non-
method CV by the fact that the CvIsMETHOD() macro will return true on it.
(Note: This macro should not be confused with the one that was previously called CvMETHOD(). That one
does not relate to the class system, and was renamed to CvNOWARN_AMBIGUOUS() to avoid this confusion.)
There is currently no extra information that needs to be stored about a method CV, so the structure does
not add any new fields.
Instances
Object instances are represented by an entirely new SV type, whose base type is "SVt_PVOBJ". This should
still be blessed into its class stash and wrapped in an RV in the usual manner for classical object.
As these are their own unique container type, distinct from hashes or arrays, the core "builtin::reftype"
function returns a new value when asked about these. That value is "OBJECT".
Internally, such an object is an array of SV pointers whose size is fixed at creation time (because the
number of fields in a class is known after compilation). An object instance stores the max field index
within it (for basic error-checking on access), and a fixed-size array of SV pointers storing the
individual field values.
Fields of array and hash type directly store AV or HV pointers into the array; they are not stored via an
intervening RV.
API
The data structures described above are supported by the following API functions.
Class Manipulation
class_setup_stash
void class_setup_stash(HV *stash);
Called by the parser on encountering the "class" keyword. It upgrades the stash into being a class and
prepares it for receiving class-specific items like methods and fields.
class_seal_stash
void class_seal_stash(HV *stash);
Called by the parser at the end of a "class" block, or for unit classes its containing scope. This
function performs various finalisation activities that are required before instances of the class can be
constructed, but could not have been done until all the information about the members of the class is
known.
Any additions to or modifications of the class under compilation must be performed between these two
function calls. Classes cannot be modified once they have been sealed.
class_add_field
void class_add_field(HV *stash, PADNAME *pn);
Called by pad.c as part of defining a new field name in the current pad. Note that this function does
not create the padname; that must already be done by pad.c. This API function simply informs the class
that the new field name has been created and is now available for it.
class_add_ADJUST
void class_add_ADJUST(HV *stash, CV *cv);
Called by the parser once it has parsed and constructed a CV for a new "ADJUST" block. This gets added to
the list stored by the class.
Field Manipulation
class_prepare_initfield_parse
void class_prepare_initfield_parse();
Called by the parser just before parsing an initializing expression for a field variable. This makes use
of a suspended compcv to combine all the field initializing expressions into the same CV.
class_set_field_defop
void class_set_field_defop(PADNAME *pn, OPCODE defmode, OP *defop);
Called by the parser after it has parsed an initializing expression for the field. Sets the defaulting
expression and mode of application. "defmode" should either be zero, or one of "OP_ORASSIGN" or
"OP_DORASSIGN" depending on the defaulting mode.
padadd_FIELD
#define padadd_FIELD
This flag constant tells the "pad_add_name_*" family of functions that the new name should be added as a
field. There is no need to call class_add_field(); this will be done automatically.
Method Manipulation
class_prepare_method_parse
void class_prepare_method_parse(CV *cv);
Called by the parser after start_subparse() but immediately before doing anything else. This prepares the
"PL_compcv" for parsing a method; arranging for the "CvIsMETHOD" test to be true, adding the $self
lexical, and any other activities that may be required.
class_wrap_method_body
OP *class_wrap_method_body(OP *o);
Called by the parser at the end of parsing a method body into an optree but just before wrapping it in
the eventual CV. This function inserts extra ops into the optree to make the method work correctly.
Object Instances
SVt_PVOBJ
#define SVt_PVOBJ
An SV type constant used for comparison with the SvTYPE() macro.
ObjectMAXFIELD
SSize_t ObjectMAXFIELD(sv);
A function-like macro that obtains the maximum valid field index that can be accessed from the
"ObjectFIELDS" array.
ObjectFIELDS
SV **ObjectFIELDS(sv);
A function-like macro that obtains the fields array directly out of an object instance. Fields can be
accessed by their field index, from 0 up to the maximum valid index given by "ObjectMAXFIELD".
OPCODES
OP_METHSTART
newUNOP_AUX(OP_METHSTART, ...);
An "OP_METHSTART" is an "UNOP_AUX" which must be present at the start of a method CV in order to make it
work properly. This is inserted by class_wrap_method_body(), and even appears before any optree fragment
associated with signature argument checking or extraction.
This op is responsible for shifting the value of $self out of the arguments list and binding any field
variables that the method requires access to into the pad. The AUX vector will contain details of the
field/pad index pairings required.
This op also performs sanity checking on the invocant value. It checks that it is definitely an object
reference of a compatible class type. If not, an exception is thrown.
If the "op_private" field includes the "OPpINITFIELDS" flag, this indicates that the op begins the
special "xhv_class_initfields_cv" CV. In this case it should additionally take the second value from the
arguments list, which should be a plain HV pointer (directly, not via RV). and bind it to the second pad
slot, where the generated optree will expect to find it.
OP_INITFIELD
An "OP_INITFIELD" is only invoked as part of the "xhv_class_initfields_cv" CV during the construction
phase of an instance. This is the time that the individual SVs that make up the mutable fields of the
instance (including AVs and HVs) are actually assigned into the "ObjectFIELDS" array. The
"OPpINITFIELD_AV" and "OPpINITFIELD_HV" private flags indicate whether it is creating an AV or HV; if
neither is set then an SV is created.
If the op has the "OPf_STACKED" flag it expects to find an initializing value on the stack. For SVs this
is the topmost SV on the data stack. For AVs and HVs it expects a marked list.
COMPILE-TIME BEHAVIOUR
"ADJUST" Phasers
During compiletime, parsing of an "ADJUST" phaser is handled in a fundamentally different way to the
existing perl phasers ("BEGIN", etc...)
Rather than taking the usual route, the tokenizer recognises that the "ADJUST" keyword introduces a
phaser block. The parser then parses the body of this block similarly to how it would parse an
(anonymous) method body, creating a CV that has no name GV. This is then inserted directly into the class
information by calling "class_add_ADJUST", entirely bypassing the symbol table.
Attributes
During compilation, attributes of both classes and fields are handled in a different way to existing perl
attributes on subroutines and lexical variables.
The parser still forms an "OP_LIST" optree of "OP_CONST" nodes, but these are passed to the
"class_apply_attributes" or "class_apply_field_attributes" functions. Rather than using a class lookup
for a method in the class being parsed, a fixed internal list of known attributes is used to find
functions to apply the attribute to the class or field. In future this may support user-supplied
extension attribute, though at present it only recognises ones defined by the core itself.
Field Initializing Expressions
During compilation, the parser makes use of a suspended compcv when parsing the defaulting expression for
a field. All the expressions for all the fields in the class share the same suspended compcv, which is
then compiled up into the same internal CV called by the constructor to initialize all the fields
provided by that class.
RUNTIME BEHAVIOUR
Constructor
The generated constructor for a class itself is an XSUB which performs three tasks in order: it creates
the instance SV itself, invokes the field initializers, then invokes the ADJUST block CVs. The
constructor for any class is always the same basic shape, regardless of whether the class has a
superclass or not.
The field initializers are collected into a generated optree-based CV called the field initializer CV.
This is the CV which contains all the optree fragments for the field initializing expressions. When
invoked, the field initializer CV might make a chained call to the superclass initializer if one exists,
before invoking all of the individual field initialization ops. The field initializer CV is invoked with
two items on the stack; being the instance SV and a direct HV containing the constructor parameters. Note
carefully: this HV is passed directly, not via an RV reference. This is permitted because both the caller
and the callee are directly generated code and not arbitrary pure-perl subroutines.
The ADJUST block CVs are all collected into a single flat list, merging all of the ones defined by the
superclass as well. They are all invoked in order, after the field initializer CV.
$self Access During Methods
When class_prepare_method_parse() is called, it arranges that the pad of the new CV body will begin with
a lexical called $self. Because the pad should be freshly-created at this point, this will have the pad
index of 1. The function checks this and aborts if that is not true.
Because of this fact, code within the body of a method or method-like CV can reliably use pad index 1 to
obtain the invocant reference. The "OP_INITFIELD" opcode also relies on this fact.
In similar fashion, during the "xhv_class_initfields_cv" the next pad slot is relied on to store the
constructor parameters HV, at pad index 2.
AUTHORS
Paul Evans
perl v5.38.2 2025-04-08 PERLCLASSGUTS(1)