"new operator" to instantiate another class as a factory?

Question

I try to use the new operator to instantiate a specific class and not the one behind the new keyword.

I try to have a kind of "factory" for an abstract class.

It seems to me that it is not possible, but lets double check ! This code compile, but the main code treat it as a Test (and not a TestImpl class)

class Test
{
public:
  virtual int testCall() { return 0; };

  static void* operator new(std::size_t);
};

class TestImpl : public Test
{
  virtual int testCall() override
  {
    return i;
  }

  int i = 15;
};

void* Test::operator new(size_t sz)
{
  return ::new TestImpl();
}

void main()
{
  Test * t = new Test(); // Call the new operator, correctly
  int i = test->testCall(); // i == 0 and not 15
}

Answer 1

Note that for every new expression, the two following things will be performed:

1. allocate memory via appropriate operator new.
2. construct the object on the memory allocated by step#1.

So operator new only allocates memory, doesn't construct the object. That means, for Test * t = new Test();, still a Test will be constructed, on the memory allocated by the overloaded operator new; even you constructed a TestImpl inside the operator new, but it'll be overwritten on the same memory soon, after the operator new finished.