Field representation

[leonerd]

How should fields of classes be represented?

There isn't a convenient precedent yet for this concept. About the closest thing that exists is the pad names elements; the SVs that exist in the PADNAMES array of a padlist. Those are just regular SVs that store the name of the lexical variable in the PV slot, and maybe have a few extra fields set on them.

A field of a class, meanwhile, would need some more data:

• The defaulting expression as a (partial) optree
• The field index within an instance representation
• The name of any associated :param attribute

[yuki-kimoto]

I think the flag for ro/wo/rw( or reader/writer) is needed.

[leonerd]

Those turn up later as a side-effect of having the various attributes applied. I haven't even begun to think about how attributes might be handled yet. Almost-certainly a very different way to the current core perl module of looking for APPLY_*_ATTRIBUTES package methods.

[leonerd]

Current Implementation (2022-08-17)

A field is a padname stored in a pad (most likely the toplevel code-less scope of its containing file), with additionally the PadnameIsFIELD() test macro returning true on that padname.

In that case, the PadnameFIELDINFO() macro will return a pointer to some additional information that only applies to fields:

    PADOFFSET  fieldix;    /* index of this field within ObjectFIELDS() array */
    HV        *fieldstash; /* original class package which added this field */

These fields:

• fieldix - the fieldix (i.e. field offset within the ObjectFIELDS() array) where values for this field are stored in each instance. Once a class is compiled, this is a constant value for all instances of this class (and, in future, instances of any subclass of it).
• fieldstash - a refcounted pointer to the stash which defines this field. Used to distinguish fields of multiple classes that may exist within the same scope (because only one toplevel pad is stored per file, and a single file might contain multiple class blocks).

API

Fields are created by passing the padadd_FIELD flag as part of a call to any of the pad_add_name_* family of functions; for example

PADOFFSET padix = pad_add_name_pvn("$x", 2, padadd_FIELD, NULL, NULL);

There is no need to additionally call class_add_field(); that will be done automatically by S_pad_alloc_name.

There is currently no introspection API for callers to ask about fields within a class. For that, code would just directly inspect the HvAUX(stash)->xhv_class_fields PADNAMELIST. Some sort of API would likely be added in future if the need arises.

[yuki-kimoto]

Do these field structure contains class variables defined by "field $foo : common", not only normal fields?