Using bitwise operators

I've been studying C# and ran accross some familiar ground from my old work in C++. I never understood the reason for bitwise operators in a real application. I've never used them and have never had in a reason to use them. I've been studying how they work; the example below shows the shift bitwise operator. What is the point of bitwise operators, their use and how they work?

Maybe I'm missing something in bitwise logic.

byte bitComp = 15;              // bitComp = 15 = 00001111b
byte bresult = (byte) ~bitComp; // bresult = 240 = 11110000b

Here's an example for the ~complement bitwise operator:

byte bitComp = 15;              // bitComp = 15 = 00001111b
byte bresult = (byte) ~bitComp; // bresult = 240 = 11110000b

---

A typical use is manipulating bits that represent mutually exclusive 'flags'.

Example from MSDN: Enumeration Types

[Flags]
enum Days2
{
    None = 0x0,
    Sunday = 0x1,
    Monday = 0x2,
    Tuesday = 0x4,
    Wednesday = 0x8,
    Thursday = 0x10,
    Friday = 0x20,
    Saturday = 0x40
}

class MyClass
{
    Days2 meetingDays = Days2.Tuesday | Days2.Thursday;

    Days2 notWednesday = ~(Days2.Wednesday);
}

See also Stack Overflow question Most common C# bitwise operations.

---

Here's an everyday bitwise-op trick not many people have discovered:

When you have an enumerated type representing a bitfield, you need to define each enum entry as a distinct bit value, as in:

enum
{
    Option1 = 1,
    Option2 = 2,
    Option3 = 4,
    Option4 = 8,
    Option5 = 16
};

but it's easy to forget that the next item in the sequence needs to be double the last number. Using bit shifting, it makes the sequence much easier to get right:

enum
{
    Option1 = 1<<0,
    Option2 = 1<<1,
    Option3 = 1<<2,
    Option4 = 1<<3,
    Option5 = 1<<4
};

---

Another typical (but I think less common) usage is to compose several numbers into one big number. An example for this can be the windows RGB macro:

#define RGB(r, g ,b)  ((DWORD) (((BYTE) (r) | ((WORD) (g) << 8)) | (((DWORD) (BYTE) (b)) << 16)))

Where you take 3 bytes and compose an integer from them the represent the RGB value.

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