C++ bitshift wraps due to conversion?

Question

Could someone explain this behavior?

#include <iostream>
using namespace std;
int main(){
    for(int i = 0; i <= 32; i++){
        unsigned long test = 1u << i;
        cout << test << endl;
    }
}

outputs:

Which makes sense until the end. Bit shift operators shouldn't wrap, but here it appears to be doing exactly that. I imagine it's because the literal '1u' is a smaller type than an unsigned long. Using '1ul' instead makes the behavior perfectly normal, so there must be something going on with the type conversion, but I'm curious to know exactly what and why!

The behavior is undefined. The shift has to be less than the size of the shiftee. I’m sure this is a duplicate, but haven’t found it. — Pete Becker, Feb 07 '19 at 18:14
Shifting a 32-bit number more than 31 steps has undefined behaviour. — molbdnilo, Feb 07 '19 at 18:15
Warm welcome into the world of [undefined behavior](https://en.wikipedia.org/wiki/Undefined_behavior). — KamilCuk, Feb 07 '19 at 18:17
@NeilButterworth, the link you marked as duplicate addresses signed integers. This post deals with unsigned integers. — R Sahu, Feb 07 '19 at 18:22
@NeilButterworth, from the post: `int a = 1; std::cout << (a << 32) << std::endl;` — R Sahu, Feb 07 '19 at 18:51

score 4 · Accepted Answer · edited Feb 08 '19 at 19:47

4

If the value of the right operand for bit-wise shift is negative or is greater or equal to the number of bits in the promoted left operand, the behavior is undefined.

From https://timsong-cpp.github.io/cppwp/n3337/expr.shift#1:

The behavior is undefined if the right operand is negative, or greater than or equal to the length in bits of the promoted left operand.

And there's no sense in explaining the behavior of undefined behavior, because it's undefined.

edited Feb 08 '19 at 19:47

R Sahu

204,454
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answered Feb 07 '19 at 18:17

wildturtle

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Not true. See https://timsong-cpp.github.io/cppwp/n3337/expr.shift#2. The behavior is well defined for signed types. – R Sahu Feb 07 '19 at 18:19
@RSahu That paragraph is not an exception to the last sentence in the preceding paragraph. – molbdnilo Feb 07 '19 at 18:28
@Rsahu My statement was only referring to the type used in the question (unsigned long), I don't know enough to speak for signed types. Perhaps I should have been more clear. – wildturtle Feb 07 '19 at 18:29

C++ bitshift wraps due to conversion?

1 Answers1