How to use bitmask?

Question

How do i use it in C++ ? when is it useful to use ?
Please give me an example of a problem where bitmask is used , how it actually works . Thanks!

Answer 1

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 == TRUE) 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.