P2590R2: "Explicit lifetime management" (start_lifetime_as) implementation

main.cpp

// How can we make this program well-defined without sacrificing efficiency? If the destination type
// is a trivially-copyable implicit-lifetime type, this can be accomplished by copying the storage
// elsewhere, using placement new of an array of byte-like type, and copying the storage back to its
// original location, then using std::launder to acquire a pointer to the newly-created object, and
// finally relying on the compiler to optimise away all the copying. However, this would be very
// verbose and hard to get right. For expressivity and optimisability, a combined operation to
// create an object of implicit-lifetime type in-place while preserving the object representation
// may be useful. This is exactly what std::start_lifetime_as is designed to do

template<class T>
T*
start_lifetime_as(void* p) noexcept
{
  // Copy the storage to a temporary location, using placement new of an array of byte-like type
  std::byte tmp[sizeof(T)];
  new (tmp) T(*reinterpret_cast<T*>(p));

  // Copy the storage back to its original location
  std::copy(tmp, tmp + sizeof(T), reinterpret_cast<std::byte*>(p));

  // then using std::launder to acquire a pointer to the newly-created object
  // and finally relying on the compiler to optimise away all the copying
  return std::launder(reinterpret_cast<T*>(p));
}

// The same operation can be performed on an array of implicit-lifetime type
template<class T>
T*
start_lifetime_as_array(void* p, size_t n) noexcept
{
  // Copy the storage to a temporary location, using placement new of an array of byte-like type
  std::byte tmp[sizeof(T) * n];
  for (size_t i = 0; i < n; i++)
  {
    new (tmp + i * sizeof(T)) T(reinterpret_cast<T*>(p)[i]);
  }

  // Copy the storage back to its original location
  std::copy(tmp, tmp + sizeof(T) * n, reinterpret_cast<std::byte*>(p));

  // then using std::launder to acquire a pointer to the newly-created object
  // and finally relying on the compiler to optimise away all the copying
  return std::launder(reinterpret_cast<T*>(p));
}

template<class T>
const T*
start_lifetime_as(const void* p) noexcept
{
  return start_lifetime_as<T>(const_cast<void*>(p));
}

template<class T>
volatile T*
start_lifetime_as(volatile void* p) noexcept
{
  return start_lifetime_as<T>(const_cast<void*>(p));
}

template<class T>
const volatile T*
start_lifetime_as(const volatile void* p) noexcept
{
  return start_lifetime_as<T>(const_cast<void*>(p));
}

template<class T>
const T*
start_lifetime_as_array(const void* p, size_t n) noexcept
{
  return start_lifetime_as_array<T>(const_cast<void*>(p), n);
}

template<class T>
volatile T*
start_lifetime_as_array(volatile void* p, size_t n) noexcept
{
  return start_lifetime_as_array<T>(const_cast<void*>(p), n);
}

template<class T>
const volatile T*
start_lifetime_as_array(const volatile void* p, size_t n) noexcept
{
  return start_lifetime_as_array<T>(const_cast<void*>(p), n);
}

// Test of the above functions
int
main()
{
  // Create an implicit-lifetime object
  std::array<std::byte, 100> record;
  std::fill(record.begin(), record.end(), std::byte(0x42));

  struct Foo
  {
    int x;
    int y;
  };

  // Create a new object of implicit-lifetime type in-place while preserving the object
  // representation
  Foo* foo = start_lifetime_as<Foo>(record.data());
  printf("%d %d", foo->x, foo->y);

  Foo* foos = start_lifetime_as_array<Foo>(record.data(), record.size() / sizeof(Foo));
  for (size_t i = 0; i < record.size() / sizeof(Foo); i++)
  {
    printf("%d %d", foos[i].x, foos[i].y);
  }

  // Test const version
  const std::array<std::byte, 100> record2 = record;
  const Foo*                       foo2    = start_lifetime_as<Foo>(record2.data());  
  printf("%d %d", foo2->x, foo2->y);

}