Comparing D vs C++ (wierd behaviour of C++)

[Caspar Kielwein]

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.