NAME¶

std::byteswap - std::byteswap

Synopsis¶

Defined in header <bit>
template< class T > (since C++23)
constexpr T byteswap( T n ) noexcept;

Reverses the bytes in the given integer value n.

std::byteswap participates in overload resolution only if T satisfies integral,
i.e., T is an integer type. The program is ill-formed if T has padding bits.

Parameters¶

n - integer value

Return value¶

An integer value of type T whose object representation comprises the bytes of that
of n in reversed order.

Notes¶

This function is useful for processing data of different endianness.

Feature-test macro Value Std Feature
__cpp_lib_byteswap 202110L (C++23) std::byteswap

Possible implementation¶

template<std::integral T>
constexpr T byteswap(T value) noexcept
{
static_assert(std::has_unique_object_representations_v<T>,
"T may not have padding bits");
auto value_representation = std::bit_cast<std::array<std::byte, sizeof(T)>>(value);
std::ranges::reverse(value_representation);
return std::bit_cast<T>(value_representation);
}

Example¶

// Run this code

#include <bit>
#include <concepts>
#include <cstdint>
#include <iomanip>
#include <iostream>

template<std::integral T>
void dump(T v, char term = '\n')
{
std::cout << std::hex << std::uppercase << std::setfill('0')
<< std::setw(sizeof(T) * 2) << v << " : ";
for (std::size_t i{}; i != sizeof(T); ++i, v >>= 8)
std::cout << std::setw(2) << static_cast<unsigned>(T(0xFF) & v) << ' ';
std::cout << std::dec << term;
}

int main()
{
static_assert(std::byteswap('a') == 'a');

std::cout << "byteswap for U16:\n";
constexpr auto x = std::uint16_t(0xCAFE);
dump(x);
dump(std::byteswap(x));

std::cout << "\nbyteswap for U32:\n";
constexpr auto y = std::uint32_t(0xDEADBEEFu);
dump(y);
dump(std::byteswap(y));

std::cout << "\nbyteswap for U64:\n";
constexpr auto z = std::uint64_t{0x0123456789ABCDEFull};
dump(z);
dump(std::byteswap(z));
}

Possible output:¶

byteswap for U16:
CAFE : FE CA
FECA : CA FE

byteswap for U32:
DEADBEEF : EF BE AD DE
EFBEADDE : DE AD BE EF

byteswap for U64:
0123456789ABCDEF : EF CD AB 89 67 45 23 01
EFCDAB8967452301 : 01 23 45 67 89 AB CD EF

See also¶

endian indicates the endianness of scalar types
(C++20) (enum)
rotl computes the result of bitwise left-rotation
(C++20) (function template)
rotr computes the result of bitwise right-rotation
(C++20) (function template)