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

[Ecstatic Coder via Digitalmars-d]

On Tuesday, 24 July 2018 at 14:08:26 UTC, Daniel Kozak wrote:
I am not C++ expert so this seems wierd to me:
#include <iostream>
#include <string>

using namespace std;

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:

import std.stdio;

void main(string[] args)
{
        char c = 0xFF;
        string sData = [c,c,c,c];
        uint i = (((((sData[0]&0xFF)*256
                                        + (sData[1]&0xFF))*256)
                                        + (sData[2]&0xFF))*256
                                        + (sData[3]&0xFF));
        if (i != 0xFFFFFFFF) { // is false - make sense
                writefln("WTF: %d", i);
        }                       
}

compiled with:
dmd -release -inline -boundscheck=off -w -of"test" "test.d"

So it is code gen bug on c++ side, or there is something wrong 
with that code.

As the C++ char are signed by default, when you accumulate 
several shifted 8 bit -1 into a char result and then store it in 
a 64 bit unsigned buffer, you get -1 in 64 bits : 
18446744073709551615.