What are bit flags ?

[The Salesman]

As the title says, just what exactly are bit flags ?
I can't seem to find a definition or explanation of bit flags on this site or in my reference book. I've finally started to learn my first programming library(SDL) through a book, however, bit flags are constantly mentioned and I don't think I'll be able to progress any further until I find out.

Any help would be appreciated, thanks

[jsmith]

In C/C++, boolean variables consume 1 byte of memory. But all that is really needed is 1 bit: 0 means false, 1 means true. Many times, especially when dealing with graphics, rather than consuming a whole byte of memory for each boolean, several booleans are combined into a single byte of memory, where each boolean uses a different bit in the byte. These are then referred to as bit flags, or bit fields.

[ropez]

This is an example, using a 8-bit unsigned integer to store 8 flags:

unsigned char options;

It is common to use larger fields, e.g. 32 bits, but I use 8 here for simplicity.

The possible options, that can be turned on or off independently are declared in an enum like this (just using some arbitrary identifiers):

enum Options {
  OpAutoRedraw    = 0x01,
  OpAntiAlias     = 0x02,
  OpPixelShader   = 0x04,
  OpVertexShader  = 0x08,
  OpFullscreen    = 0x10,
  OpDaylight      = 0x20
  // ...
};

Note how each option is given a specific value. These values are carefully picked to match each bit in the 8-bit variable:

// 0x01 ==   1 == "00000001"
// 0x02 ==   2 == "00000010"
// 0x04 ==   4 == "00000100"
// 0x08 ==   8 == "00001000"
// 0x10 ==  16 == "00010000"
// 0x20 ==  32 == "00100000"
// 0x40 ==  64 == "01000000"
// 0x80 == 128 == "10000000" 

Now, each flag can be set independently, by using the bitwise OR operator:

options = OpAutoRedraw | OpVertexShader | OpFullscreen;
// options == 0x01 | 0x08 | 0x10 == "00011001" 

And can be tested using the bitwise AND operator:

if (options & OpAutoRedraw) {} // true
if (options & OpAntiAlias) {} // false 

I hope this helps!