How can I use a bitmask?

How do I use it in C++? When is it useful to use?

What would be an example of a problem where a bitmask is used to see how it actually works?

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最佳答案

Bit masking is "useful" to use when you want to store (and subsequently extract) different data within a single data value.

An example application I've used before is imagine you were storing colour RGB values in a 16 bit value. So something that looks like this:

RRRR RGGG GGGB BBBB

You could then use bit masking to retrieve the colour components as follows:

  const unsigned short redMask   = 0xF800;
const unsigned short greenMask = 0x07E0;
const unsigned short blueMask  = 0x001F;


unsigned short lightGray = 0x7BEF;


unsigned short redComponent   = (lightGray & redMask) >> 11;
unsigned short greenComponent = (lightGray & greenMask) >> 5;
unsigned short blueComponent =  (lightGray & blueMask);
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Briefly, a bitmask helps to manipulate the position of multiple values. There is a good example here;

Bitflags are a method of storing multiple values, which are not mutually exclusive, in one variable. You've probably seen them before. Each flag is a bit position which can be set on or off. You then have a bunch of bitmasks #defined for each bit position so you can easily manipulate it:

    #define LOG_ERRORS            1  // 2^0, bit 0
#define LOG_WARNINGS          2  // 2^1, bit 1
#define LOG_NOTICES           4  // 2^2, bit 2
#define LOG_INCOMING          8  // 2^3, bit 3
#define LOG_OUTGOING         16  // 2^4, bit 4
#define LOG_LOOPBACK         32  // and so on...


// Only 6 flags/bits used, so a char is fine
unsigned char flags;


// Initialising the flags,
// Note that assigning a value will clobber any other flags, so you
// it should generally only use the = operator when initialising variables.
flags = LOG_ERRORS;
// Sets to 1 i.e. bit 0


// Initialising to multiple values with OR (|)
flags = LOG_ERRORS | LOG_WARNINGS | LOG_INCOMING;
// sets to 1 + 2 + 8 i.e. bits 0, 1 and 3


// Setting one flag on, leaving the rest untouched
// OR bitmask with the current value
flags |= LOG_INCOMING;


// Testing for a flag
// AND with the bitmask before testing with ==
if ((flags & LOG_WARNINGS) == LOG_WARNINGS)
...


// Testing for multiple flags
// As above, OR the bitmasks
if ((flags & (LOG_INCOMING | LOG_OUTGOING))
== (LOG_INCOMING | LOG_OUTGOING))
...


// Removing a flag, leaving the rest untouched
// AND with the inverse (NOT) of the bitmask
flags &= ~LOG_OUTGOING;


// Toggling a flag, leaving the rest untouched
flags ^= LOG_LOOPBACK;


**

WARNING: DO NOT use the equality operator (i.e. bitflags == bitmask) for testing if a flag is set - that expression will only be true if that flag is set and all others are unset. To test for a single flag you need to use & and == :

**

if (flags == LOG_WARNINGS) //DON'T DO THIS
...
if ((flags & LOG_WARNINGS) == LOG_WARNINGS) // The right way
...
if ((flags & (LOG_INCOMING | LOG_OUTGOING)) // Test for multiple flags set
== (LOG_INCOMING | LOG_OUTGOING))
...

You can also search C++ Tricks.

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Let's say I have 32-bit ARGB value with 8-bits per channel. I want to replace the alpha component with another alpha value, such as 0x45

unsigned long alpha = 0x45
unsigned long pixel = 0x12345678;
pixel = ((pixel & 0x00FFFFFF) | (alpha << 24));

The mask turns the top 8 bits to 0, where the old alpha value was. The alpha value is shifted up to the final bit positions it will take, then it is OR-ed into the masked pixel value. The final result is 0x45345678 which is stored into pixel.

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Bitmasks are used when you want to encode multiple layers of information in a single number.

So (assuming unix file permissions) if you want to store 3 levels of access restriction (read, write, execute) you could check for each level by checking the corresponding bit.

rwx
---
110

110 in base 2 translates to 6 in base 10.

So you can easily check if someone is allowed to e.g. read the file by and'ing the permission field with the wanted permission.

Pseudocode:

PERM_READ = 4
PERM_WRITE = 2
PERM_EXEC = 1


user_permissions = 6


if ((user_permissions & PERM_READ) == PERM_READ) then
// this will be reached, as 6 & 4 is true
fi

You need a working understanding of binary representation of numbers and logical operators to understand bit fields.


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