The Slots in an Object from a Formal Class
The definition of the class specifies all slots directly and indirectly defined for that class. Each slot has a name and an associated class. Extracting a slot returns an object from that class. Setting a slot first coerces the value to the specified slot and then stores it.
Unlike general attributes, slots are not partially matched, and asking for (or trying to set) a slot with an invalid name for that class generates an error.
The @ extraction operator and slot function themselves do no checking against the class definition, simply matching the name in the object itself. The replacement forms do check (except for slot in the case check=FALSE). So long as slots are set without cheating, the extracted slots will be valid.
Be aware that there are two ways to cheat, both to be avoided but with no guarantees. The obvious way is to assign a slot with check=FALSE. Also, slots in R are implemented as attributes, for the sake of some back compatibility. The current implementation does not prevent attributes being assigned, via attr<-, and such assignments are not checked for legitimate slot names.
Note that the "@" operators for extraction and replacement are primitive and actually reside in the base package.
The replacement versions of "@" and slot() differ in the computations done to coerce the right side of the assignment to the declared class of the slot. Both verify that the value provided is from a subclass of the declared slot class. The slot() version will go on to call the coerce method if there is one, in effect doing the computation as(value, slotClass, strict = FALSE). The "@" version just verifies the relation, leaving any coerce to be done later (e.g., when a relevant method is dispatched).
In most uses the result is equivalent, and the "@" version saves an extra function call, but if empirical evidence shows that a conversion is needed, either call as() before the replacement or use the replacement version of slot().
An object from a formally defined class.
The name of the slot. The operator takes a fixed name, which can be unquoted if it is syntactically a name in the language. A slot name can be any non-empty string, but if the name is not made up of letters, numbers, and ., it needs to be quoted (by backticks or single or double quotes).
In the case of the slot function, name can be any expression that evaluates to a valid slot in the class definition. Generally, the only reason to use the functional form rather than the simpler operator is because the slot name has to be computed.
A new value for the named slot. The value must be valid for this slot in this object's class.
In the replacement version of slot, a flag. If TRUE, check the assigned value for validity as the value of this slot. User's code should not set this to FALSE in normal use, since the resulting object can be invalid.
either the name of a class (as character string), or a class definition. If given an argument that is neither a character string nor a class definition, slotNames (only) uses class(x) instead.