Is there ANY chance we can fix the bitwise operator precedence rules?
Don
nospam at nospam.com
Fri Jun 18 14:03:24 PDT 2010
In the comments for bug 4077, "Bugs caused by bitwise operator precedence"
it was asked why C gave & with lower precedence than ==, when it is
unintuitive and a frequent source of bugs.
I was quite shocked to find that the reason is backwards compatibility
with the B programming language.
Denis Ritchie says (http://cm.bell-labs.com/cm/cs/who/dmr/chist.html):
--------------------
At the suggestion of Alan Snyder, I introduced the && and || operators
to make the mechanism [[short circuit evaluation]] more explicit.
Their tardy introduction explains an infelicity of C's precedence rules.
In B one writes
if (a==b & c) ...
to check whether a equals b and c is non-zero; in such a conditional
expression it is better that & have lower precedence than ==. In
converting from B to C, one wants to replace & by && in such a
statement; to make the conversion less painful, we decided to keep the
precedence of the & operator the same relative to ==, and merely split
the precedence of && slightly from &. Today, it seems that it would have
been preferable to move the relative precedences of & and
==, and thereby simplify a common C idiom: to test a masked value
against another value, one must write
if ((a&mask) == b) ...
where the inner parentheses are required but easily forgotten.
-----------------------------------
Tragic. Can we end this madness?
Could we give & | ^ the same precedence as ==, making
(a & mask == b) an error, just as (a < b == c) is rejected?
That way we could lay this common bug to rest. I've personally lost days
of my life to this one. It's a bug 100% of the time.
Or is too late to break backwards compatibility with B ?
BTW I think this a great cautionary tale about the dangers of rating
backwards compatibility too highly.
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