Is there any way to get the memory size (usage) of a Lisp variable/value?

Question

Basically sizeof for lists, hashmaps, etc., with nested structures.

I could of course disable garbage collection and then compare numbers from memory usage functions before and after allocation, getting vaguely informative but imprecise numbers—due to probably also having created intermediate vars.

But maybe there's a method that involves less approximate guesswork and more straightforward calculation?

Answer 1

Is there any way to get the memory size/usage of a variable?

Emacs 28 added a more fine-grained memory usage reporting command than was previously available:

** New command 'memory-report'.
This command opens a new buffer called "*Memory Report*" and gives a
summary of where Emacs is using memory currently.

One of its subroutines is memory-report-object-size:

(defun memory-report-object-size (object)
  "Return the size of OBJECT in bytes."
  (unless memory-report--type-size
    (memory-report--garbage-collect))
  (memory-report--object-size (make-hash-table :test #'eq) object))

For example:

(let ((table1 (make-hash-table :size 1))
      (table2 (make-hash-table :size 2)))
  (mapcar #'memory-report-object-size
          (list (cons nil nil)
                (list nil nil)
                table1
                table2
                (cons table1 table2)
                (list table1 table2))))
;; => (16 32 32 48 96 112)