slot-value

(slot-value object 'slot-name)

Reads a slot of a defclass instance, and is also a setf-able place. A literal quoted symbol slot name resolves to the slot's fixed position at compile/expansion time. On the interpreter a computed slot name (a variable or expression) is also accepted and resolves the slot at runtime by name; on the compiled backends it is an error. With a literal name, a slot name used at different positions by two unrelated classes is rejected as ambiguous (within one inheritance chain positions always agree — prefer :accessor/:reader functions, which are per-class). Reading does not type-check the object, like defstruct accessors.

A slot name no class in the program declares is a run-time error on every backend (The slot X is missing), not a compile-time one -- the same as reading it on the interpreter, and catchable with handler-case.