pointer increment

[Maarten Litmaath]

wolfgang at ruso.UUCP (Wolfgang Deifel) writes:
\maart at cs.vu.nl (Maarten Litmaath) writes:
\> I wrote:
\>\...
\>\    ((char*)ptr)++ ;
\>\Now ptr becomes 1 ( = sizeof(char) ) more.
\
\>Aaaaaaarrrrrgghh!  Wayne Throop's Sacred Mission isn't finished yet...
\ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
\ 
\ Thank god that you are born as expert...

There's no need to feel p*ssed off, Wolfgang; I wasn't trying to insult you
or something.
Of course I should have added a detailed explanation, but I figured someone
else would supply that.
Having read your article I remembered Wayne Throop's 1985 (1986?) crusade
against "the vile heresy which claimeth that ((char*)ptr)++ will increment
ptr" (free after Henry Spencer's Ten Commandments for C Programmers).

Allright.  A compiler that allows the abovementioned construct is wrong for
two reasons:

1)	It allows the `++' operator to be applied to an Rvalue expression;
	only Lvalue expressions may be operand of an increment operator, e.g.

		x++
		a[i]++

	This isn't so strange:

		x++
	
	is equivalent to

		x = x + 1

	which doesn't make sense for arbitrary (Rvalue) expressions.
2)	It increments the wrong variable; a cast is equivalent to an
	assignment to an invisible temporary variable (with the usual
	restrictions and conversions):

		foo	x;

		... (bar) x ...

	becomes

		foo	x;
		bar	cast_tmp;	/* `invisible' temp variable */

		... (cast_tmp = x) ...
	
	If we write the latter expression as

		(cast_tmp = x, cast_tmp)

	it's clearly `cast_tmp' which should be incremented (if a `++'
	operator is appended), were this possible at all; definitely NOT `x'.
-- 
"rot H - dD/dt = J, div D = rho, div B = 0, |Maarten Litmaath @ VU Amsterdam:
  rot E + dB/dt = 0" and there was light.   |maart at cs.vu.nl, mcvax!botter!maart