Make 64 bit masks: set bits [a, b] to 1's and others 0's

make-64-bit-mask.cc

// C++ version 1
#include <iostream>
#include <string>
#include <bitset>
using namespace std; // yeah I know it's better to use "ns::foo" instead of declaring "using namespace ns" at the top...
                     // but this file is just a toy, right. Live a little.

int main_old()
{
  int a = 31, b = 63;
  uint64_t mask = (((uint64_t) -1 >> ((uint64_t)63 - (b))) & ~(((uint64_t)1 << (a)) - 1));
  bitset<64> maskbits(mask);
  cout << maskbits.to_string() << endl;
}

// C++ version 2
#include <iostream>
#include <string>
#include <bitset>
using namespace std;

/**
 * Class templates: UnsignedIntTypeSelector
 * Usage: typename UnsignedIntTypeSelector<64>::type fooReturnUint64(); <=> uint64_t fooReturnUint64();
 * ---------------------------
 * Select uint*_t based on number of bits.
 */
template <unsigned short> struct UnsignedIntTypeSelector;
template <> struct UnsignedIntTypeSelector<64> { typedef uint64_t type; };
template <> struct UnsignedIntTypeSelector<32> { typedef uint32_t type; };
template <> struct UnsignedIntTypeSelector<16> { typedef uint16_t type; };
template <> struct UnsignedIntTypeSelector<8>  { typedef uint8_t type; };

/**
 * Function template: getBitMask<>()
 * Usage: uint64_t mask = getBitMask<64>(60, 63) => 111100..0 (high bits 63~60 set to 1).
 * ---------------------------
 * Set bits [lo, hi] to 1's and others 0's, as long as 0 <= lo <= hi <= (NUM_BITS - 1).
 */
template <unsigned short NUM_BITS>
constexpr typename UnsignedIntTypeSelector<NUM_BITS>::type getBitMask(uint64_t lo, uint64_t hi) {
    typedef typename UnsignedIntTypeSelector<NUM_BITS>::type return_type;
    return ((return_type) -1 >> ((return_type)NUM_BITS - 1 - (hi))) & ~(((return_type)1 << (lo)) - 1);
}

int main()
{
  uint8_t mask = getBitMask<8>(3, 5);
  bitset<8> maskbits(mask);
  cout << maskbits.to_string() << endl; // print "00111000"
}