eieio: Slot Options
3.2 Slot Options
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The SLOT-LIST argument to ‘defclass’ is a list of elements where each
element defines one slot. Each slot is a list of the form
(SLOT-NAME :TAG1 ATTRIB-VALUE1
:TAG2 ATTRIB-VALUE2
:TAGN ATTRIB-VALUEN)
where SLOT-NAME is a symbol that will be used to refer to the slot.
:TAG is a symbol that describes a feature to be set on the slot.
ATTRIB-VALUE is a lisp expression that will be used for :TAG.
Valid tags are:
‘:initarg’
A symbol that can be used in the argument list of the constructor
to specify a value for this slot of the new instance being created.
A good symbol to use for initarg is one that starts with a colon
‘:’.
The slot specified like this:
(myslot :initarg :myslot)
could then be initialized to the number 1 like this:
(myobject :myslot 1)
Making New Objects.
‘:initform’
An expression used as the default value for this slot.
If ‘:initform’ is left out, that slot defaults to being unbound.
It is an error to reference an unbound slot, so if you need slots
to always be in a bound state, you should always use an ‘:initform’
specifier.
Use ‘slot-boundp’ to test if a slot is unbound (
Predicates). Use ‘slot-makeunbound’ to set a slot to being
unbound after giving it a value (Accessing Slots).
The value passed to initform used to be automatically quoted.
Thus,
:initform (1 2 3)
will use the list as a value. This is incompatible with CLOS
(which would signal an error since 1 is not a valid function) and
will likely change in the future, so better quote your initforms if
they’re just values.
‘:type’
An unquoted type specifier used to validate data set into this
slot. (cl)Type Predicates. Here are some examples:
‘symbol’
A symbol.
‘number’
A number type
‘my-class-name’
An object of your class type.
‘(or null symbol)’
A symbol, or ‘nil’.
‘:allocation’
Either :class or :instance (defaults to :instance) used to specify
how data is stored. Slots stored per instance have unique values
for each object. Slots stored per class have shared values for
each object. If one object changes a :class allocated slot, then
all objects for that class gain the new value.
‘:documentation’
Documentation detailing the use of this slot. This documentation
is exposed when the user describes a class, and during
customization of an object.
‘:accessor’
Name of a generic function which can be used to fetch the value of
this slot. You can call this function later on your object and
retrieve the value of the slot.
This options is in the CLOS spec, but is not fully compliant in
EIEIO.
‘:writer’
Name of a generic function which will write this slot.
This options is in the CLOS spec, but is not fully compliant in
EIEIO.
‘:reader’
Name of a generic function which will read this slot.
This options is in the CLOS spec, but is not fully compliant in
EIEIO.
‘:custom’
A custom :type specifier used when editing an object of this type.
See documentation for ‘defcustom’ for details. This specifier is
equivalent to the :type spec of a ‘defcustom’ call.
This options is specific to Emacs, and is not in the CLOS spec.
‘:label’
When customizing an object, the value of :label will be used
instead of the slot name. This enables better descriptions of the
data than would usually be afforded.
This options is specific to Emacs, and is not in the CLOS spec.
‘:group’
Similar to ‘defcustom’’s :group command, this organizes different
slots in an object into groups. When customizing an object, only
the slots belonging to a specific group need be worked with,
simplifying the size of the display.
This options is specific to Emacs, and is not in the CLOS spec.
‘:printer’
This routine takes a symbol which is a function name. The function
should accept one argument. The argument is the value from the
slot to be printed. The function in ‘object-write’ will write the
slot value out to a printable form on ‘standard-output’.
The output format MUST be something that could in turn be
interpreted with ‘read’ such that the object can be brought back in
from the output stream. Thus, if you wanted to output a symbol,
you would need to quote the symbol. If you wanted to run a
function on load, you can output the code to do the construction of
the value.
‘:protection’
This is an old option that is not supported any more.
When using a slot referencing function such as ‘slot-value’, and
the value behind SLOT is private or protected, then the current
scope of operation must be within a method of the calling object.
This protection is not enforced by the code any more, so it’s only
useful as documentation.
Valid values are:
‘:public’
Access this slot from any scope.
‘:protected’
Access this slot only from methods of the same class or a
child class.
‘:private’
Access this slot only from methods of the same class.
This options is specific to Emacs, and is not in the CLOS spec.
Info Catalog
eieio: Inheritance
eieio: Building Classes
eieio: Class Options