Padding and undefined behavior

padding_ub.cc

// When compiled (macOS, 64 bits) with g++-8 -O3 -std=c++14  ub.cc  -Wall -Wextra:
// 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0
// 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
// Notice the '1' coming from nowhere :-)

// Note that clang++ produces:
// 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
// 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

#include <cstdint>
#include <iostream>

struct padding_auto
{
  std::uint32_t field0;
  // padding added here by the compiler
  std::uint64_t field1;
};

int main()
{
  const auto f0 = padding_auto{0, 0};
  const auto f1 = padding_auto{0, 0};

  const auto ptr0 = reinterpret_cast<const char*>(&f0);
  for (auto i = 0ul; i < sizeof(padding_auto); ++i)
  {
    std::cout << +ptr0[i] << ' ';
  }

  std::cout << '\n';

  const auto ptr1 = reinterpret_cast<const char*>(&f1);
  for (auto i = 0ul; i < sizeof(padding_auto); ++i)
  {
    std::cout << +ptr1[i] << ' ';
  }
}