Generic integer byte swap function for C++11, C++23 already has one https://en.cppreference.com/w/cpp/numeric/byteswap

cpp_11_generic_byte_swap.hpp

#pragma once

#include <type_traits>

template <typename Integer,
          std::enable_if_t<std::is_integral<Integer>::value, bool> = true>
static Integer swap_bytes(Integer value)
{
    Integer out;
    for (int i = 0; i < sizeof(value); i++)
    {
        reinterpret_cast<char *>(&out)[sizeof(value) - i - 1] = reinterpret_cast<char *>(&value)[i];
    }
    return out;
}