error: conversion from ‘unique_ptr<GreenStack,default_delete<GreenStack>>’ to non-scalar type ‘unique_ptr<Stack,default_delete<Stack>>’ requested

Question

I am trying to assign a unique_ptr holding a derived class pointer to a unique_ptr holding a base class pointer. However, I am receiving the following error:

error: conversion from ‘unique_ptr<GreenStack,default_delete<GreenStack>>’ to non-scalar type ‘unique_ptr<Stack,default_delete<Stack>>’ requested

Code snippet is below.

class GreenStack;    
class Stack {
 public:
  explicit Stack(double initial_weight) : weight_(initial_weight) {}
  static std::unique_ptr<Stack> makeGreenStack(double initial_weight)
  {
    //std::unique_ptr<Stack> box =  std::make_unique<Stack>(initial_weight);
    std::unique_ptr<Stack> green_box_01 =  std::make_unique<GreenStack>(initial_weight);
    return std::move(green_box_01);
  }
  bool operator<(const Stack& rhs) const { return weight_ < rhs.weight_; }
  virtual ~Stack() = default;

 protected:
  double weight_;
};

class GreenStack:public Stack
{
  public:
  explicit GreenStack(double initial_weight): Stack(initial_weight){}
  ~GreenStack() = default;

};

Please guide to resolve this error.

Answer 1

Define Stack::makeGreenStack(double) after defining GreenStack to be derived from Stack.

The compiler will then know that a std::unique_ptr<Stack> can be initialized from a std::unique_ptr<GreenStack>

class GreenStack;    
class Stack {
 public:
  explicit Stack(double initial_weight) : weight_(initial_weight) {}
  static std::unique_ptr<Stack> makeGreenStack(double initial_weight);

  bool operator<(const Stack& rhs) const { return weight_ < rhs.weight_; }
  virtual ~Stack() = default;

 protected:
  double weight_;
};

class GreenStack:public Stack
{
  public:
  explicit GreenStack(double initial_weight): Stack(initial_weight){}
  ~GreenStack() = default;

};

std::unique_ptr<Stack> Stack::makeGreenStack(double initial_weight)
{
    std::unique_ptr<Stack> green_box_01 =  std::make_unique<GreenStack>(initial_weight);
    return green_box_01;
}