c++ - "new operator" to instantiate another class as a factory? -

i try use new operator instantiate specific class , not 1 behind new keyword.

i try have kind of "factory" abstract class.

it seems me not possible, lets double check ! code compile, main code treat test (and not 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 = 15; };  void* test::operator new(size_t sz) {   return ::new testimpl(); }  void main() {   test * t = new test(); // call new operator, correctly   int = test->testcall(); // == 0 , not 15 } 

note every new expression, 2 following things performed:

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

so operator new allocates memory, doesn't construct object. means, test * t = new test();, still test constructed, on memory allocated overloaded operator new; constructed testimpl inside operator new, it'll overwritten on same memory soon, after operator new finished.