Bitset in C++

std::bitset Member Functions

The std::bitset class provides several useful member functions for manipulating & querying the bits in the set.  Some of the most commonly used ones are:

1. test(pos): Returns true if the bit at position pos is set (1), false otherwise.
    
2. set(pos, val = true): Sets the bit at position pos to val (default is 1).
    
3. reset(pos): Resets the bit at position pos to 0.
    
4. flip(pos): Flips the bit at position pos (0 becomes 1, 1 becomes 0).
    
5. all(): Returns true if all bits are set, false otherwise.
    
6. any(): Returns true if at least one bit is set, false otherwise.
    
7. none(): Returns true if no bits are set, false otherwise.
    
8. count(): Returns the number of bits that are set.
    
9. size(): Returns the total number of bits in the bitset.
    
10. to_string(): Returns a string representation of the bitset.
     
11. to_ulong(): Returns an unsigned long integer representation of the bitset.
     
12. to_ullong(): Returns an unsigned long long integer representation of the bitset.

For example: 

#include <iostream>
#include <bitset>
int main() {
   std::bitset<8> bs("10101010");
   std::cout << bs << std::endl;
   std::cout << bs.count() << std::endl;
   std::cout << bs.test(0) << std::endl;
   std::cout << bs.test(1) << std::endl;
   bs.flip();
   std::cout << bs << std::endl;
   bs.set();
   std::cout << bs << std::endl;
   return 0;
}

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Output

10101010
4
0
1
01010101
11111111

In this example, we create a bitset bs with the binary string "10101010." We then print out the bitset and the number of set bits (4) and test the first two bits. We flip all the bits using flip(), set all the bits using set(), and print out the modified bitset each time.

std::bitset operators

std::bitset Operators

The std::bitset class overloads several operators to make it easy to perform bitwise operations & comparisons on bitsets. Here are the most commonly used ones:

1. &: Bitwise AND operator. Returns a new bitset that is the bitwise AND of the operands.
 

2. |: Bitwise OR operator. Returns a new bitset that is the bitwise OR of the operands. 
 

3. ^: Bitwise XOR operator. Returns a new bitset that is the bitwise XOR of the operands.
 

4. ~: Bitwise NOT operator. Returns a new bitset that is the bitwise complement of the operand.
 

5. <<: Left shift operator. Shifts the bits in the bitset to the left by the specified number of positions.
 

6. >>: Right shift operator. Shifts the bits in the bitset to the right by the specified number of positions.
 

7. ==, !=: Equality & inequality operators. Return true if the bitsets are equal/not equal, false otherwise.
 

8. <, <=, >, >=: Comparison operators. Compare the bitsets lexicographically.

For example 

#include <iostream>
#include <bitset>
int main() {
   std::bitset<4> bs1("1010");
   std::bitset<4> bs2("1100");
   std::cout << (bs1 & bs2) << std::endl;
   std::cout << (bs1 | bs2) << std::endl;
   std::cout << (bs1 ^ bs2) << std::endl;
   std::cout << ~bs1 << std::endl;
   std::cout << (bs1 << 1) << std::endl;
   std::cout << (bs1 >> 1) << std::endl;
   std::cout << (bs1 == bs2) << std::endl;
   std::cout << (bs1 != bs2) << std::endl;
   std::cout << (bs1 < bs2) << std::endl;
   return 0;
}

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Output

1000
1110
0110
0101
0100
0101
0
1
1

In this example, we create two bitsets bs1 & bs2, & perform various bitwise operations on them using the overloaded operators. We print out the results of each operation to see how they work.

The bitwise AND, OR & XOR operators (&, |, ^) return new bitsets that are the result of applying the corresponding operation to each pair of bits in the operands. The bitwise NOT operator (~) returns a new bitset that is the complement of the operand (0s become 1s & vice versa).

The left & right shift operators (<<, >>) shift the bits in the bitset by the specified number of positions, filling in with 0s. The equality, inequality & comparison operators (==, !=, <, <=, >, >=) compare the bitsets lexicographically, i.e., by comparing the bits from left to right.

Difference between std::bitset and std::vector<bool> and an array of bool

Frequently Asked Questions

Can std::bitset be resized during runtime?

No, the size of std::bitset is fixed at compile-time and cannot be changed during runtime.

Is std::vector<bool> more memory-efficient than an array of bool?

Yes, std::vector<bool> is more memory-efficient than an array of bool because it packs bits into memory, while an array of bool stores each boolean value as a separate byte.

Which container provides the best performance for bitwise operations?

std::bitset provides the best performance for bitwise operations due to its compact storage and optimized member functions.

Conclusion

In this article, we discussed the std::bitset class in C++, which provides an efficient way to store and manipulate fixed-size sequences of bits. We covered the syntax, member functions, and operators of std::bitset, and provided examples to show its implementation. We also compared std::bitset with std::vector<bool> and arrays of bool, discussing their differences in storage, size, performance, functionality, and ease of use. 

You can also check out our other blogs on Code360.