On 17 Jan 2007 16:36:04 -0600, Gabriel Dos Reis <[EMAIL PROTECTED]> wrote:
Paolo Carlini <[EMAIL PROTECTED]> writes:
| Joe Buck wrote:
|
| >In the case of the containers, we are asserting/relying on the fact that
| >the pointer difference is zero or positive. But this has become a
| >widespread idiom: people write their own code in the STL style. If STL
| >code now has to be fixed to silence warnings, so will a lot of user code.
| >
| Good point. About it, we should also take into account the recent
| messages from Martin, pointing out that many C++ front-ends do not
| warn for signed -> unsigned.
I just built firefox (CVS) with GCC mainline. The compiler spitted
avalanches of non-sensical warning about conversions signed ->
unsigned may alter values, when in fact the compiler knows that
such things cannot happen.
First, let's recall that GCC supports only 2s complement targets.
Second, a conversion from T to U may alter value if a round trip is
not the identity function. That is, there exists a value t in T
such that the assertion
assert (T(U(t)) == t)
fails.
I think it warns if U(t) != t in a mathematical sense (without promoting
to the same type for the comparison), so it warns as (unsigned)-1 is
not "-1".
I agree this warning is of questionable use and should be not enabled
with -Wall.
Richard.
