On Tuesday, 24 July 2018 at 14:08:26 UTC, Daniel Kozak wrote:
I am not C++ expert so this seems wierd to me:
(...)
int main(int argc, char **argv)
{
char c = 0xFF;
std::string sData = {c,c,c,c};
unsigned int i = (((((sData[0]&0xFF)*256
+ (sData[1]&0xFF))*256)
+ (sData[2]&0xFF))*256
+ (sData[3]&0xFF));
if (i != 0xFFFFFFFF) { // it is true why?
// this print 18446744073709551615 wow
std::cout << "WTF: " << i << std::endl;
}
return 0;
}
compiled with:
g++ -O2 -Wall -o "test" "test.cxx"
when compiled with -O0 it works as expected
Vs. D: ....
So it is code gen bug on c++ side, or there is something wrong
with that code.
Signedness of char in C++ is platform dependent.
See https://en.cppreference.com/w/cpp/language/types "char"
You seem to be running into "signed overflow is undefined
behaviour" shenanigans.
with all optimizations clang gives a different result than gcc.
https://godbolt.org/g/Dz5djj
Generally use unsigned char (or std::byte) when char means
"memory".
And prefer a std::vector<unsigned char> to std::string in these
cases as well.
