c++17引入的函数介绍 std::launder

struct X { const int n; };
union U { X x; float f; };
void tong() {
  U u = 1;
  u.f = 5.f;               // OK, creates new subobject of 'u'
  X *p = new (&u.x) X {2}; // OK, creates new subobject of 'u'
  assert(p->n == 2);       // OK
  assert(*std::launder(&u.x.n) == 2); // OK

  // undefined behavior, 'u.x' does not name new subobject
  assert(u.x.n == 2);
}

template <typename T>
FOLLY_NODISCARD inline T* launder(T* in) noexcept {
#if FOLLY_HAS_BUILTIN(__builtin_launder) || __GNUC__ >= 7
  // The builtin has no unwanted side-effects.
  return __builtin_launder(in);
#elif __GNUC__
  // This inline assembler block declares that `in` is an input and an output,
  // so the compiler has to assume that it has been changed inside the block.
  __asm__("" : "+r"(in));
  return in;
#elif defined(_WIN32)
  // MSVC does not currently have optimizations around const members of structs.
  // _ReadWriteBarrier() will prevent compiler reordering memory accesses.
  _ReadWriteBarrier();
  return in;
#else
  static_assert(
      false, "folly::launder is not implemented for this environment");
#endif
}