Abstract class and unique pointer

Question

I have the following error in my code:

error: allocating an object of abstract class type 'Material'

I don't know how to handle this case.

I'm aware that std::make_unique performs an allocation, so it can't allocate the object of type Material, but I don't know how to correct it.

#include <iostream>
#include <memory>

struct Material
{
  Material() = default;
  virtual int get_color() const = 0;
};

struct Basic : public Material
{
  Basic() = default;
  virtual int get_color() const override
  {
    return 1;
  }
};

struct Mix : public Material
{
  Mix(const Material& mat1, const Material& mat2)
    : mat1_(std::make_unique<Material>(mat1))
    , mat2_(std::make_unique<Material>(mat2))
  {}

  virtual int get_color() const override
  {
    return mat1_->get_color() + mat2_->get_color();
  }     
private:
  std::unique_ptr<Material> mat1_;
  std::unique_ptr<Material> mat2_;
};

int main()
{
  auto mix = Mix(Basic(), Basic());
  std::cout << mix.get_color() << '\n';
}

Answer 1

This call:

std::make_unique<Material>(mat1)

tries to create an instance of class Material, it is irrelevant what type mat1 has. You seem to need method clone() in your class:

class Material {
...
    virtual std::unique_ptr<Material> clone() const = 0;
};

then Mix ctor would be:

Mix(const Material& mat1, const Material& mat2)
    : mat1_(mat1.clone())
    , mat2_(mat2.clone())
  {}

and you need to implement clone() in every derived class:

struct Basic : public Material
{
  Basic() = default;

  virtual std::unique_ptr<Material> clone() const override
  {
      return std::make_unique<Basic>( *this ); 
  }

  virtual int get_color() const override
  {
    return 1;
  }
};

Answer 2

You can use a templated constructor to arrange for constructing the right types without needing a clone method:

#include <iostream>
#include <memory>

struct Material {
    Material() = default;
    virtual int get_color() const = 0;
};

struct Basic : Material {
    Basic() = default;
    int get_color() const override {
        return 1;
    }
};

struct Mix : Material {
    template<typename M1, typename M2>
    Mix(const M1& mat1, const M2& mat2)
        : mat1_{std::make_unique<M1>(std::move(mat1))}
        , mat2_{std::make_unique<M2>(std::move(mat2))}
    {} 

    int get_color() const override {
        return mat1_->get_color() + mat2_->get_color();
    }

private:
    std::unique_ptr<Material> mat1_;
    std::unique_ptr<Material> mat2_;
};

int main() {
    auto mix = Mix(Basic(), Basic());
    std::cout << mix.get_color() << '\n';
}

Note that this works only if you send instance of a material with a known type.