Unexpected behaviour using unsigned char in comparison

[Bob Plantz]

On 1/9/2011 10:00 AM, Falk S. wrote:
> Hi,
>
> in the following program the return value of the test2uc() function is 
> always false, however I would expect it to be true.
> #include <iostream>
> using namespace std;
>
> bool test2uc() {
>        typedef unsigned char uchar;
>        uchar mask = ~(uchar(0)) << 1;
>        return (mask | uchar(true)) == (~ uchar(0));
> }
>
> int main(int argc, char** args) {
>        cout << test2uc() << endl;
>        return 0;
> }
>
> Compiled with -ansi -Wall -g -gdwarf-2 -pedantic
> g++ (GCC) 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)
> g++.exe (GCC) 4.5.0 (MingW)
> on Windows 7
>
> I get a "warning: comparison is always false due to limited range of 
> data type" when compiling with g++ a c++ file, which I do not understand.
> If I change the type of uchar to char, i.e. signed char, I get the 
> expected result.
>
> Any ideas?
> bye,
> Falk
Not sure what you're trying to do, but I find it a bit odd that you're doing bit operations until you do (mask | uchar(true)).

Did you intend to use "||"?

--Bob