isc30/stack-factory-puzzle

Stack Factory Puzzle

Stack Factory Puzzle

When instances of classes derived from the same class need to be created based on certain conditions,
they are created on the heap. The puzzle is about how to create them on the stack.

In main, code between comments Begin End creates instances of classes X0 or X1 or X2 (depends on i) on the heap
(similar to a factory function which creates instances of classes on the heap and returns pointer to their base class).
Console output of the program is:

X0::X0 X0::Process: A   X0::~X0
X1::X1 X1::Process: ABC	X1::~X1
X2::X2 X2::Process: 123	X2::~X2

How to modify code between the comments Begin End, that it doesn't call new and console output is the same.

#include <iostream>
#include <string>
#include <memory>

struct X
{
  friend struct X0;
  friend struct X1;
  friend struct X2;
  
  virtual ~X() {}
  virtual void Process() = 0;
  
private:
  X() {}
};

struct X0 final: public X
{
  X0() { std::cout << "X0::X0 "; }
  ~X0() { std::cout << "\tX0::~X0\n"; }
    
  void Process() override { std::cout << "X0::Process: " << c_; }
  
  char c_ = 'A';
};

struct X1 final: public X
{
  X1() { std::cout << "X1::X1 "; }
  ~X1() { std::cout << "\tX1::~X1\n"; }
    
  void Process() override { std::cout << "X1::Process: " << s_; }
  
  std::string s_ = "ABC";
};

struct X2 final: public X
{
  X2() { std::cout << "X2::X2 "; }
  ~X2() { std::cout << "\tX2::~X2\n"; }
    
  void Process() override { std::cout << "X2::Process: " << i_; }
  
  int i_ = 123;
};

int main()
{
  for (auto i = 0; i < 3; i++) 
  {
    X* pX = nullptr;

    // Begin 
    if (i == 0)
      pX = new X0;
    else if (i == 1)
      pX = new X1;
    else 
      pX = new X2;
      
    std::unique_ptr<X> x{pX};    
    // End

    pX->Process();
  }

  return 0;
}