c++ - Weird result after assigning 2^31 to a signed and unsigned 32-bit integer variable

Question

As the question title reads, assigning 2^31 to a signed and unsigned 32-bit integer variable gives an unexpected result.

Here is the short program (in C++), which I made to see what's going on:

#include <cstdio>
using namespace std;

int main()
{
    unsigned long long n = 1<<31;
    long long n2 = 1<<31;  // this works as expected
    printf("%llu
",n);
    printf("%lld
",n2);
    printf("size of ULL: %d, size of LL: %d
", sizeof(unsigned long long), sizeof(long long) );
    return 0;
}

Here's the output:

MyPC / # c++ test.cpp -o test
MyPC / # ./test
18446744071562067968      <- Should be 2^31 right?
-2147483648               <- This is correct ( -2^31 because of the sign bit)
size of ULL: 8, size of LL: 8

I then added another function p(), to it:

void p()
{
  unsigned long long n = 1<<32;  // since n is 8 bytes, this should be legal for any integer from 32 to 63
  printf("%llu
",n);
}

On compiling and running, this is what confused me even more:

MyPC / # c++ test.cpp -o test
test.cpp: In function ‘void p()’:
test.cpp:6:28: warning: left shift count >= width of type [enabled by default]
MyPC / # ./test 
0
MyPC /

Why should the compiler complain about left shift count being too large? sizeof(unsigned long long) returns 8, so doesn't that mean 2^63-1 is the max value for that data type?

It struck me that maybe n*2 and n<<1, don't always behave in the same manner, so I tried this:

void s()
{
   unsigned long long n = 1;
   for(int a=0;a<63;a++) n = n*2;
   printf("%llu
",n);
}

This gives the correct value of 2^63 as the output which is 9223372036854775808 (I verified it using python). But what is wrong with doing a left shit?

A left arithmetic shift by n is equivalent to multiplying by 2ⁿ (provided the value does not overflow)

-- Wikipedia

The value is not overflowing, only a minus sign will appear since the value is 2^63 (all bits are set).

I'm still unable to figure out what's going on with left shift, can anyone please explain this?

PS: This program was run on a 32-bit system running linux mint (if that helps)

Answer 1

On this line:

unsigned long long n = 1<<32;

The problem is that the literal 1 is of type int - which is probably only 32 bits. Therefore the shift will push it out of bounds.

Just because you're storing into a larger datatype doesn't mean that everything in the expression is done at that larger size.

So to correct it, you need to either cast it up or make it an unsigned long long literal:

unsigned long long n = (unsigned long long)1 << 32;
unsigned long long n = 1ULL << 32;