C++ big endian vs little endian

Table of contents

Byte significant

• LSB - Less significant byte
  • Any change to will cause a small change in this number
• MSB - Most significant byte
  • Any change to will cause a huge change in this number

  // uint32_t value = 1958516461;
  uint32_t value = 0x01234567;
  cout << fmt::format("Value printed in hex:  {:08x}", value);

Any object in C++ can be reinterpreted as an array of bytes. If you want to actually make a copy of the bytes into a separate array, you can use std::copy:

  unsigned char bytes[sizeof value];
  std::copy(
      static_cast<const unsigned char *>(static_cast<const void *>(&value)),
      static_cast<const unsigned char *>(static_cast<const void *>(&value)) +
          sizeof value,
      bytes);

  // Print
  cout << "\nBytes in memory (1): ";
  for (const auto &byte : bytes) {
    cout << fmt::format("{:02x} ", byte);
  }

  // Print
  cout << "\nBytes in memory (2): " << hex;
  copy(begin(bytes), end(bytes), std::ostream_iterator<int>(cout, " "));

When above program is run on little endian machine, gives “67
  //-- When above program is run on little endian machine, gives “67
45 23 01” as output, while if it is run on big endian machine,
  //-- 45 23 01” as output, while if it is run on big endian machine,
gives “01 23 45 67” as output.
  //-- gives “01 23 45 67” as output.

How to detect endianness in C

  {
    unsigned int i = 1;
    // i: 01 00 00 00 on little endian machines
    // i: 00 00 00 01 on big endian machines
    // c: ^^
    char *c = (char *)&i;
    if (*c) {
      cout << "\nlittle-endian";
    } else {
      cout << "\nbig-endian";
    }
    // This code is contributed by rathbhupendra
  }

How to detect endianness in C++

  // #include <bit>
  if constexpr (std::endian::native == std::endian::big) {
    std::cout << "\nbig-endian";
  } else if constexpr (std::endian::native == std::endian::little) {
    std::cout << "\nlittle-endian";
  } else {
    std::cout << "\nmixed-endian";
  }

Possible output

Value printed in hex:  01234567
Bytes in memory (1): 67 45 23 01 
Bytes in memory (2): 67 45 23 1 
little-endian
little-endian