std::optional: How, when, and why - C++ Team Blog

using T = /* some object type */;

struct S {
  bool is_initialized = false;
  alignas(T) unsigned char maybe_T[sizeof(T)];

  void construct_the_T(int arg) {
    assert(!is_initialized);
    new (&maybe_T) T(arg);
    is_initialized = true;
  }

  T& get_the_T() {
    assert(is_initialized);
    return reinterpret_cast<T&>(maybe_T);
  }

  ~S() {
    if (is_initialized) {
      get_the_T().~T(); // destroy the T
    }
  }

  // ... lots of code ...
};