Bitwise (Bitshift) operations on 64-bit integers in C++

Question

I'm trying to deal with bitboards, which requires me to set a particular bit in a 64-bit unsigned integer. To set bit i, I perform a bitwise OR with the bitboard in question, with a left shifted number.

#include <stdint.h>
uint64_t kings = 0ULL; // Also tried unsigned long long int before.
kings |= 1 << i;

It works fine from bit 0 to bit 31, but fails to work for bits 32 to 63. I suspect it's because the evaluation of the right side happens to be in a 32-bit integer. Therefore, I tried a temporary variable.

uint64_t temp = 0ULL;
temp |= 1 << i;

Perhaps it still evaluates the right hand side as a 32-bit integer, or it's some other problem I cannot figure out. To output the integer, I'm using std::bitset<64>. For example:

uint64_t kings = 0ULL;
kings |= 1 << 3;
kings |= 1 << 59;

• Expected decimal value: 576460752303423496
• Actual: 8

std::bitset<64> x(kings);
std::cout << x;

• Bit value: 0000000000000000000000000000000000000000000000000000000000001000

Clearly, only kings |= 1 << 3; worked correctly.

In summary, what is the issue with bits 32 to 63 and how can I work around it?

Answer 1

You need to use 1LL as 64 bit value before you use shift operator << to get 64 bit result:

#include <stdint.h>
uint64_t kings = 0ULL; 
kings |= 1ULL << i;