Date:	Sun, 20 Nov 94 02:01:20 JST
From:	Stephen Turnbull <turnbull AT shako DOT sk DOT tsukuba DOT ac DOT jp>
To:	davis AT amy DOT tch DOT harvard DOT edu ()
Cc:	dolan AT fnoc DOT navy DOT mil, gordon AT spot DOT colorado DOT edu, djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:	printf or floating point error?????

   Date: Sat, 19 Nov 1994 04:46:56 -0500
   From: davis AT amy DOT tch DOT harvard DOT edu ("John E. Davis")

   >A cast does not say "convert this value on the right to this type
   >and assign it to the recipient on the left", a cast says "for
   >right now, the bit pattern contained in the piece of _storage_ at
   >the location indicated by the object being cast IS this type, use
   >it as this type and pass the appropriate value under that
   >interpretation into the recipient on the left", not at all the
   >same thing, it turns out.

   I do not think that this statement is correct. 

The statement is simply wrong.  A cast is a type conversion, pure and
simple.   In C, I don't think that you can cast an object to a
structure at all, but I don't have a manual handy.  In C++ v1.x
(according to Stroustrup) you can cast a class (struct, union) object
to another type only if there is a conversion operator defined for
that conversion.
    The construct that Kent was thinking of, I guess, is a union.

   Consider:

      float x, y;
      long i = 10;

      /* assume sizeof (float) == sizeof (long) */

      x = (float) i;
      y = *(float *) &i;

   Here `x' will have the value of 10.0 whereas `y' has the bit
   pattern that you re suggesting above.  `y' will will almost
   certainly not be 10.0 and `x' will not have the bit pattern of `i'.
   That is, the float cast above will definitely modify the bit
   pattern of i.

This example is bogus because

      x = i;

will work without the cast, and it means (in K&R C)

      x = (float) (double) i;

because of the "usual arithmetic conversions," at least as I
understand them.  (And in K&R C the value of "x = i" is a double!)  I
am fairly sure my statements about C++ and C hold for C++ v2 and ANSI
C respectively, but don't have the references at hand.
    --Steve

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