Why does the left shift on a unsigned int happens from the 16th bit? - c++11

Question

I am trying to put the values from the vector into the int.
Given vector :'1','0','1','1','1','0','1','1','1','0','1','1','1','0','1','1' :
Expected output (binary representation for the variable out):
00000000000000001011101110111011.
However, I am getting the following output:
10111011101110110000000000000000
Notice: the insertion begun at the 16bit from right end instead of beginning from the leftmost bit
#include<vector>
#include<iostream>
int main() {
std::vector<unsigned char> test = {'1','0','1','1','1','0','1','1','1','0','1','1','1','0','1','1'};
std::vector<unsigned int> out(1);
int j = 0;
for (int i =0; i < test.size(); i++) {
out[j] = out[j] << 1;
if (test[i] == '1') {out[j] |=0x1;}
}
j++;
for (int p = 0; p < j; p++) {
for (int k = 0; k<32; k++ ) {
std::cout << !!((out[p]<<k)&0x8000);
}
std::cout << std::endl;
}
std::cout << "Size Of:" << sizeof(int);
return 0;
}

Answer 1

The reason why this happens is that you are using a wrong constant for the mask: 0x8000 has its 16-bit set, while you probably meant to use 0x80000000 with the 32-nd bit set. To avoid mistakes like that it's best to construct masks with shifts, for example
(1 << 31)
This expression is evaluated at compile time, so the result is the same as if you computed the constant yourself.
Note that both 0x8000 and 0x80000000 constants are system-dependent. Moreover, 0x80000000 assumes 32-bit int, which is not guaranteed.
A better approach would be shifting the number right instead of left, and masking with 1.

Answer 2

The block of code creating out[j] works just fine.
Your problem is in the output block, due to use of 0x8000. Whenever k >= 16, the low 16 bits will be zero, guaranteeing that 0x8000 is zero.

Answer 3

Your code seems overly complicated to me. Here's my version of a C program that transforms a string of 1's and 0's into an int and one going from int to string.
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argv);
int main (int argc, char **argv) {
char str [] = "1010101010101010";
int x;
int out;
for (x=0;x<16;x++) {
if (str[x] == '1') {
out |= (1 << x);
}
}
printf("%d", out) ;
}
and
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argv);
int main (int argc, char **argv) {
char str [] = "1010101010101010";
int in = 21845;
char out[17] = {0};
for (x=0;x<16;x++) {
if (in & (1<<x)) {
out[x] = '1';
}
else {
out[x] = '0';
}
}
printf("%s", out) ;
}