C++ bitset interesting facts

Bitset is a container in C++ Standard Template Library for dealing with data at the bit level. 

1. A bitset stores bits (elements with only two possible values: 0 or 1). We can however get the part of a string by providing positions to bitset constructor (Positions are with respect to string position from left to right)

Example:

C++

// C++ program to demonstrate that we can get part of a
// bit string in bitset.
#include <bitset>
#include <string>
#include <iostream>

int main()
{
  std::string bit_string = "110010";
  std::bitset<8> b1(bit_string);             // [0, 0, 1, 1, 0, 0, 1, 0]

  // string from position 2 till end
  std::bitset<8> b2(bit_string, 2);      // [0, 0, 0, 0, 0, 0, 1, 0]

  // string from position 2 till next 3 positions
  std::bitset<8> b3(bit_string, 2, 3);   // [0, 0, 0, 0, 0, 0, 0, 1]

  std::cout << b1 << '\n' << b2 << '\n' << b3 << '\n';

  return 0;
}

Output:

00110010
00000010
00000001

2. We can construct a bitset using the characters in the std::basic_string _str. An optional starting position _pos and length _n can be provided, as well as characters denoting alternate values for set (_one) and unset (_zero) bits. 

Syntax:

std::bitset b1(str, pos, n, zero, one);
str    : string used to initialize the bitset
pos    : a starting offset into str
n    : number of characters to use from str
zero    : alternate character for unset bits in str
one    : alternate characters for set bits in str 

• If _pos > str.size(), this constructor throws std::out_of_range.
• If any characters examined in _str is not zero or one, it throws std::invalid_argument.

C++

// C++ program to demonstrate that we can construct bitset using
// alternate characters for set and unset bits.
#include <bitset>
#include <string>
#include <iostream>

int main()
{
    // string constructor using custom zero/one digits
    std::string alpha_bit_string = "aBaaBBaB";
    std::bitset<8> b1(alpha_bit_string, 0, alpha_bit_string.size(),
                    'a', 'B');         // [0,1,0,0,1,1,0,1]

    std::cout << b1 << '\n';
}

Output:

01001101

3. Constructs an object of class bitset, initializing the N bits to values that correspond to the characters provided in a c-style character string of zeros and ones. You call the constructor without casting the string into a string type. It also has two optional parameters, _Zero and _One, which indicate what character in _Str is to be interpreted to mean a 0 bit and a 1 bit, respectively.

C++

#include <bitset>
#include <iostream>

int main()
{
    // char* constructor using custom digits
    std::bitset<8> b1("XXXXYYYY", 8, 'X', 'Y'); // [0, 0, 0, 0, 1, 1, 1, 1]
    std::cout << b1 << '\n';
}

Output:

00001111

Bitset Operations

1. std::bitset::to_string()
Converts the contents of the bitset to a string. Uses zero to represent bits with value of false and one to represent bits with value of true. The resulting string contains N characters with the first character corresponds to the last (N-1th) bit and the last character corresponding to the first bit. Also, we can pass the characters used to print true and false value through the parameters.

Example:

C++

// C++ program to demonstrate that we can convert contents
// of bitset to a string.
#include <iostream>
#include <bitset>

int main()
{
    std::bitset<8> b(42);
    std::cout << b.to_string() << '\n'
              << b.to_string('*') << '\n'
              << b.to_string('O', 'X') << '\n';
}

Output:

00101010
**1*1*1*
OOXOXOXO

2. std::bitset::to_ulong():
Converts the contents of the bitset to an unsigned long integer. The first bit of the bitset corresponds to the least significant digit of the number and the last bit corresponds to the most significant digit. Function throws std::overflow_error if the value cannot be represented in unsigned long.

Example:

C++

// C++ program to demonstrate that we can get value of bitset
// as  unsigned long integer.
#include <iostream>
#include <bitset>

int main()
{
    std::bitset<5> b(5);
    std::cout << b.to_ulong() << '\n';
}

Output:

5