Instantiate unique_ptr to derived class from vector of base class where derived class has been stored

Question

Consider following piece of code:

struct  Fruit
{
   Fruit() {}
   virtual ~Fruit() {}       
   std::string name;
};

struct Banana : public Fruit
{
   std::string color;
};

struct Pineapple : public Fruit
{
   int weight;
};

here is my main() :

int main()
{
    std::vector<std::unique_ptr<Fruit>> product;
    product.push_back(std::unique_ptr<Banana>(new Banana)); //product[0] is a Banana
    product.emplace_back(new Pineapple);

    // I need to acess the "color" member of product[0]
    std::unique_ptr<Banana> b = std::move(product[0]); // this doesn't work, why?
    auto c = b->color;
}

in product[0] I store a unique_ptr to Banana, why can't I assign it to a banana unique_ptr ?

Answer 1

You don't want ownership transfer, so cast only the pointer:

auto& banana = dynamic_cast<Banana&>(*product[0]);
auto c = banana.color;

dynamic_cast might be replaced by static_cast if you are really sure that Fruitis really a Banana. In case your are wrong, static_cast would lead to UB whereas you can check validity with dynamic_cast (exception with cast to reference or null pointer with cast to pointer).