Chapter 26. Boost.DynamicBitset

The library Boost.DynamicBitset provides the class boost::dynamic_bitset, which is used like std::bitset. The difference is that the number of bits for std::bitset must be specified at compile time, whereas the number of bits for boost::dynamic_bitset is specified at run time.

To use boost::dynamic_bitset, include the header file boost/dynamic_bitset.hpp.

#include <boost/dynamic_bitset.hpp>
#include <iostream>

int main()
{
  boost::dynamic_bitset<> db{3, 4};

  db.push_back(true);

  std::cout.setf(std::ios::boolalpha);
  std::cout << db.size() << '\n';
  std::cout << db.count() << '\n';
  std::cout << db.any() << '\n';
  std::cout << db.none() << '\n';

  std::cout << db[0].flip() << '\n';
  std::cout << ~db[3] << '\n';
  std::cout << db << '\n';
}

boost::dynamic_bitset is a template that requires no template parameters when instantiated; default types are used in that case. More important are the parameters passed to the constructor. In Example 26.1, the constructor creates db with 3 bits. The second parameter initializes the bits; in this case, the number 4 initializes the most significant bit – the bit on the very left.

The number of bits inside an object of type boost::dynamic_bitset can be changed at any time. The member function push_back() adds another bit, which will become the most significant bit. Calling push_back() in Example 26.1 causes db to contain 4 bits, of which the two most significant bits are set. Therefore, db stores the number 12.

You can decrease the number of bits by calling the member function resize(). Depending on the parameter passed to resize(), bits will either be added or removed.

boost::dynamic_bitset provides member functions to query data and access individual bits. The member functions size() and count() return the number of bits and the number of bits currently set, respectively. any() returns true if at least one bit is set, and none() returns true if no bit is set.

To access individual bits, use array syntax. A reference to an internal class is returned that represents the corresponding bit and provides member functions to manipulate it. For example, the member function flip() toggles the bit. Bitwise operators such as operator~ are available as well. Overall, the class boost::dynamic_bitset offers the same bit manipulation functionality as std::bitset.

Like std::bitset, boost::dynamic_bitset does not support iterators.