CODEWITHSUNDEEP
c++bit-manipulation
BO.LI
BO.LI

Reputation: 315

How does the | operator work?

I am feeling confused with the | operator in C++. I have code to transfer the data which read from a MCU. The high 8 bits are seperated from low 8 bits. And the data(the BUF in code) store the complement number. So I use (BUF[1] << 8) | BUF[0] to get my raw data. However, the result is a bit strange. For example, the now code get d1=-84. As the pic shows, why doesn't the | operator get results as I want? enter image description here

#include <cstdlib>
#include <cstdio>
#include <cmath>
#include<iostream>

int main() {
    signed char BUF[2];
    BUF[0] = -84;
    BUF[1] = -2;
    short d1;
    d1 = (BUF[1] << 8) | BUF[0];  // | operator
    std::cout << d1 << std::endl;
    std::cin.get();
}

Upvotes: 0

Views: 114

Answers (2)

Bathsheba
Bathsheba

Reputation: 234715

The behaviour on left shifting a negative number is undefined, and prior to C++14 you are vulnerable to different complementing schemes for a signed char. (From C++14 signed char and char if it's signed must be 2's complement.)

For an easy life, write merely

BUF[1] * 256 + BUF[0]

noting that the type of this expression is an int.

Upvotes: 3

Lundin
Lundin

Reputation: 213862

You cannot left-shift negative numbers, doing so invokes undefined behavior: anything can happen. Similarly, right-shifting negative numbers is also a bad idea, since that can result in either arithmetic or logical right shift.

You have to cast the variable to unsigned type, shift, then convert back. For example:

d1 = ((uint32_t)BUF[1] << 8) ...

Upvotes: 5

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