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CS253 Bit Manipulation

Refresher

C++ bit-manipulation tools:

bitwise and (&), or (|), exclusive-or (^)
shift left (<<), shift right (>>)
assignment operator versions (&=, |=, ^=, <<=, >>=)
octal constant (0377)
hexadecimal constant (0x3FF)
C++14 binary constant (0b1010110111)
C++14 digit separator: 0b1'010'110'111 or 0x0123'4567'89ab'cdef or 123'456'789

Displaying values in other bases

// Still no binary output‽
cout << (0b10000001 ^ 03) << '\n';
cout << (127 & 0xfe) << '\n';
cout << hex << (127 & 0xfe) << '\n';
cout << oct << (127 & 0xfe) << '\n';

I/O manipulators such as hex and oct are “sticky”; they persist until changed.

Precedence matters

cout << (1|2) << '\n';

However, the binary bitwise operators are low precedence:

cout << 1|2 << '\n';

c.cc:1: error: no match for 'operator|' in '
   std::cout.std::basic_ostream<char>::operator<<(1) | (2 << 10)' (operand 
   types are 'std::basic_ostream<char>' and 'int')

Setting a bit

int n = 32;
cout << hex << n << '\n';
cout << hex << (n|02) << '\n';

20
22

Or, using assignment and C++14’s binary literals:

int n = 32;
cout << hex << n << '\n';
n |= 0b10;
cout << hex << n << '\n';

20
22

Clearing bits

cout << hex << 126 << '\n';
cout << hex << (126 & ~0xF) << '\n';

7e
70

Testing bits

for (char c='A'; c<'N'; c++)
    if (c & 0x01)
        cout << "'" << c << "' (" << int(c) << ") is odd.\n";
    else
        cout << "'" << c << "' (" << int(c) << ") is even.\n";

'A' (65) is odd.
'B' (66) is even.
'C' (67) is odd.
'D' (68) is even.
'E' (69) is odd.
'F' (70) is even.
'G' (71) is odd.
'H' (72) is even.
'I' (73) is odd.
'J' (74) is even.
'K' (75) is odd.
'L' (76) is even.
'M' (77) is odd.

Shifting

int n = 42;
cout << oct << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 2;
cout << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 1;
cout << n << '\n';

Badness

// Don’t shift too far:
cout << (1<<32) << '\n';

c.cc:2: warning: left shift count >= width of type
0

// Don’t shift negatively:
cout << (0b1000 << -1) << '\n';

c.cc:2: warning: left shift count is negative
4

// Shifting negative values right is implementation-defined:
cout << (-1 >> 1) << '\n';

-1

CS253: Software Development with C++

Spring 2019

Bit Manipulation