bit masking tricks

1Setting a bit
2Use the bitwise OR operator (|) to set a bit.
3
4number |= 1UL << n;
5That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.
6
7Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.
8
9Clearing a bit
10Use the bitwise AND operator (&) to clear a bit.
11
12number &= ~(1UL << n);
13That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.
14
15Toggling a bit
16The XOR operator (^) can be used to toggle a bit.
17
18number ^= 1UL << n;
19That will toggle the nth bit of number.
20
21Checking a bit
22You didn't ask for this, but I might as well add it.
23
24To check a bit, shift the number n to the right, then bitwise AND it:
25
26bit = (number >> n) & 1U;
27That will put the value of the nth bit of number into the variable bit.
28
29Changing the nth bit to x
30Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:
31
32number ^= (-x ^ number) & (1UL << n);
33Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.
34
35To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.
36
37number ^= (-(unsigned long)x ^ number) & (1UL << n);
38or
39
40unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
41number ^= (-newbit ^ number) & (1UL << n);
42It's generally a good idea to use unsigned types for portable bit manipulation.
43
44or
45
46number = (number & ~(1UL << n)) | (x << n);
47(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.
48
49It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.