Question

In GNU Emacs, what is [C-tab]? Consider:

(version)
"GNU Emacs 23.2.1 (i386-mingw-nt5.1.2600)
 of 2010-05-08 on G41R2F1"

(defun key-binding-test ()
  (interactive)
  (insert " key-binding-test called "))

For a single letter control character, a character constant must be used in the vector:

(global-set-key [C-l] 'key-binding-test) ; does not work
(global-set-key [?\C-l] 'key-binding-test) ; works

?\C-l can be evaluated in the *scratch* buffer:

?\C-l
12

However to bind C-Tab:

(global-set-key [?\C-tab] 'key-binding-test) ;does not work
    ; Debugger entered--Lisp error: (invalid-read-syntax "?")
(global-set-key [C-tab] 'key-binding-test) ; works

When I try to evalulate C-tab though:

C-tab ; Debugger entered--Lisp error: (void-variable C-tab)

Comparing the evaluation of the vectors:

[?\C-l] ; input
[12]    ; result

[C-tab] ; input
[C-tab] ; result

(aref [C-tab] 0) ; input
C-tab            ; result, but C-tab can not be evaulated further.

Answer 1

[C-tab] is a vector, see the manual for vectors. In there you will find that a vector is considered constant for evaluation (i.e. it evaluates to itself).

So [C-tab] evaluates to [C-tab], a vector of one element, the symbol C-tab, which you can extract like so

(aref [C-tab] 0)

Added in response to the first comment.

Another vector is:

[some-symbol another (a list of things) 9]

it has length 4

(length [some-symbol another (a list of things) 9])

It contains two symbols some-symbol and another, a list (a list of things) and an integer 9.

C-tab is a symbol just like some-symbol and another in the examples above, they have no value unless their value cell is set to something.