# Bit header file in C++20

#### Software Engineering C++

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The <bit> header was included in C++20.

# Introduction

The <bit> header was included in the C++20. This header provides components such as types and functions to access, manipulate and process individual bits and bit sequences. It is included in the <numeric> library.

### Binary representation

Binary representation uses 0's and 1's to represent values. For example an integer 52 is represented as 110100.
knowing the binary representation can be very useful. Certain properties of binary representation have many applications in programming. For example, A integral power of 2 always has a single 1 in its representation, left-shifting the representation gives the effect of multiplying by 2 and similarly right-shifting will divide by 2.

Left shifting a 1 bit thus allows multiplication by 2.
1 << 0 = 0b1 = 1 = 20
1 << 1 = 0b10 = 2 = 21
1 << 2 = 0b100 = 4 = 22
...
1 << k = 0b100... = 2k
Thus we can see that a power of 2 always has a single 1 bit in its representation.

For numbers the usual representation has a most significant bit (MSB) on the left hand side and the least significant bit (LSB) on the right hand side.
for example: 10 is represented in binary as 1010 (0b1010), here the leftmost 1 bit is the MSB and the rightmost 0 bit is the LSB.

### Byte ordering

A byte is made up of 8 bits. When more than one byte of memory is to be stored in memory, there are two orderings possible: big endian, where most significant byte is stored first, and the little endian, where least significant byte is stored first.

# Types

The header has one type: endian, which defines the endianness or the byte ordering. A value of std::endian::little means little-endian is used, std::endian::big means big-endian is used. The value of std::endian::native is used to find the endianness of the current implementation. All of these are implementation defined.

In C++, endian is defined as an enum

if constexpr (endian::native == endian::big) {
cout << "big-endian" << endl;
}
else if constexpr (endian::native == endian::little) {
cout << "little-endian"  << endl;
}
else {
cout << "mixed-endian"  << endl;
}


# Functions

## std::bit_cast

This function allows us to reinterpret an object of one type as another. This is useful when we need to convert the underlying representation of an object.

double f64v = 19880124.0;
auto u64v = bit_cast<std::uint64_t>(f64v);

cout << fixed << f64v << "f64 == 0x" << hex << u64v << "u64" << endl;
// 19880124.000000f64 == 0x4172f58bc0000000u64


## std::has_single_bit

This function checks if the value has a single 1 bit in its binary representation. Having a single bit implies that the value is a power of two. Thus this function returns true if the value is an integral power of two.

Consider the following example where we check if the value has a single 1 bit

cout << boolalpha;
uint8_t a = 0b00001101;
uint8_t b = 0b00000010;
cout << has_single_bit(a) << endl; // false
cout << has_single_bit(b) << endl; // true


## std::bit_ceil, std::bit_floor

The std::bit_ceil function finds the binary ceiling of a value. It finds a power of two that is not smaller than the value.

The std::bit_floor function finds the binary floor of a value. It finds a power of two that is not larger than the value.

Example:

uint8_t a = 0b00000011; // 3

cout << (unsigned)(bit_ceil(a)) << endl; // 4
cout << (unsigned)(bit_floor(a)) << endl; // 2


## std::bit_width

The number of bits in an integer is given by the formula 1+floor(log2(x)). This function allows us to find this value. This value is the length of x in binary when there are no preceding 0's. For example the value of 3 (0b00000011) would require 2 bits. For the input value 0, the function returns 0.

example:

unsigned int a = 55; // 110111
cout << bit_width(a) << endl; // 6


## std::rotl, std::rotr

std::rotl performs a bitwise left rotation and returns the result, while the std::rotr performs a bitwise right rotation. Each of these functions take the value and a number that indicates the number of positions to rotate.

example:

std::uint8_t a = 0b00011101;
cout << bitset<8>( rotl(a, 2) ) << endl; // 0b01110100
cout << bitset<8>( rotr(a, 3) ) << endl; // 0b10100011


## Counting consecutive bits

These functions count the number of consecutive bits starting from the LSB or MSB position. The input needs to be an unsigned number.

### Count starting from MSB

std::countl_zero counts the number of consecutive 0 bits, starting from the most significant bit and std::countl_one counts the number of consecutive 1 bits, starting from the most significant bit.

### Count starting from LSB

std::countr_zero counts the number of consecutive 0 bits, starting from the least significant bit and std::countr_one counts the number of consecutive 1 bits, starting from the least significant bit.

Consider the example:

uint8_t x = 0b00101111;
uint8_t y = 0b11010110;

cout << countl_zero(x) << endl; // 2
cout << countl_one(x) << endl; // 0

cout << countr_zero(y) << endl; // 1
cout << countr_one(y) << endl; // 0


## std::popcount

This function counts the number of 1 bits in an unsigned integer.
for example:

uint8_t i = 0b01011101;
cout << popcount(i) << endl; // 5


# Conclusion

The <bit> header, included in the C++20, provides components to access, manipulate and process individual bits and bit sequences. std::endian allows us to determine the endianness of the implementation. The header also include many other functions that allow us to reinterpret an object, check if a number is a power of two, count bits, etc.

#### Rohit Topi

Intern at OpenGenus | B.Tech Computer Science Student at KLS Gogte Institute Of Technology Belgaum | Contributor to OpenGenus book: "Binary Tree Problems: Must for Interviews and Competitive Coding"

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