I want to create a shared ptr vector of different objects

Question

You have 2 classes Cats and Dogs and need to create a vector of shared pointers which stores the data from those 2 classes. Hints: polymorphism and keep in mind that classes can have similar fields.

So this is what I've done until now. I want to insert in that shared_ptr vector all the info Cats and Dogs classes have, but I don't know how. I only managed to insert in that vector the data from the base class.

#include <iostream>
#include <vector>
#include <memory>

class Animal
{
protected:
    int tip;
    std::string name;
    int age;
public:
    Animal(int t, std::string n, int a): tip(t), name(n), age(a) {}

    friend std::ostream& operator<<(std::ostream& os, const Animal& a)
    {
        os << "Name: " << a.name << std::endl;
        os << "Age: " << a.age << std::endl;

        return os;
    }
};

class Cats: public Animal
{
    std::string race;
    std::string pref_food;
public:
    Cats(int t = 0, std::string n = "", int a = 0, std::string r = "", std::string mnprf = ""):
        Animal(t, n, a), race(r), pref_food(mnprf) {}

    friend std::ostream& operator<<(std::ostream& os, const Cats& c)
    {
        // auto n = static_cast<Animal> (c);
        os << "Name: " << c.name << std::endl;
        os << "Age: " << c.age << std::endl;
        os << "race: " << c.race << std::endl;
        os << "Fav food: " << c.pref_food << std::endl;

        return os;
    }
};

class Dog: public Animal
{
    std::string disease;
    std::string master;
public:
    Dog(int t = 1, std::string n = "", int a = 0, std::string b = "", std::string s = "" ):
        Animal(t, n, a), disease(b), master(s) {}

    friend std::ostream& operator<<(std::ostream& os, const Dog& d)
    {
        os << "Name: " << d.name << std::endl;
        os << "Age: " << d.age << std::endl;
        os << "disease: " << d.disease << std::endl;
        os << "master: " << d.master << std::endl;

        return os;
    }
};

template<typename T>
void add(std::vector<std::shared_ptr<Animal>>& vec, const T& a)
{
    auto newptr = std::make_shared<Animal>(a);
    vec.push_back(newptr);
}

int main()
{
    std::vector<std::shared_ptr<Animal>> Animals;
    Dog d(1,"Rex", 12, "idk", "Oscar");
    Cats c(0,"Meaw", 11, "Sfinx", "Catfood");
    add(Animals,d);
    add(Animals,c);

    for(auto i: Animals)
    {
        std::cout << *i;
    }
}

remeber what you learned about polymorphism.... It is unclear why you first create `Dog d(...)` and `Cats c(...)` on the stack and then copy them to construct the smart pointers. Use `shared_ptr` from the start — 463035818_is_not_an_ai, Mar 25 '20 at 18:14
Change `std::make_shared` to `std::make_shared` would be a good start — Alan Birtles, Mar 25 '20 at 18:21
Why are you creating so many objects and taking the detour through the `add` function? You don't need anything more complicated than `Animals.push_back(std::make_shared(1,"Rex", 12, "idk", "Oscar");`. If you really need `add`, the type of the object want to copy is `T`, not `Animal`. — molbdnilo, Mar 25 '20 at 18:23
If you're going to store pointers to a base class such as `Animal`, that class should have a virtual destructor. — Spencer, Mar 25 '20 at 19:17

Remy Lebeau · Accepted Answer · 2020-03-25T20:30:21.660

There are a few problems with your code:

Animal lacks any virtual methods. At the very least, it needs a virtual destructor, so that the destructors of Cats and Dog are called correctly when shared_ptr<Animal> calls delete on its held Animal* pointer.
add() is creating an instance of Animal specifically, regardless of T. So your vector contains only real Animal objects. add() needs to create an instance of T instead. A std::shared_ptr<T> can be assigned to a std::shared_ptr<Animal> when T derives from Animal. Of course, add() is redundant, main() can just create and add the new objects directly to its vector without using add() at all.
When main() calls operator<< on an Animal, it will not call the operator<< defined by Cats or Dog, only the operator<< defined by Animal. This can be fixed by having operator<< in Animal call a virtual method that Cats and Dog override. There is no need to define operator<< in derived classes when the base class also has an operator<<.

Try this instead:

#include <iostream>
#include <vector>
#include <memory>

class Animal
{
protected:
    int tip;
    std::string name;
    int age;

public:
    Animal(int t, std::string n, int a): tip(t), name(n), age(a) {}
    virtual ~Animal() {}

    virtual void print(std::ostream& os) const
    {
        os << "Name: " << name << std::endl;
        os << "Age: " << age << std::endl;
    }

    friend std::ostream& operator<<(std::ostream& os, const Animal& a)
    {
        a.print(os);
        return os;
    }
};

class Cat : public Animal
{
    std::string race;
    std::string pref_food;

public:
    Cat(int t = 0, std::string n = "", int a = 0, std::string r = "", std::string mnprf = ""):
        Animal(t, n, a), race(r), pref_food(mnprf) {}

    void print(std::ostream& os) const override
    {
        Animal::print(os);
        os << "race: " << race << std::endl;
        os << "Fav food: " << pref_food << std::endl;
    }
};

class Dog : public Animal
{
    std::string disease;
    std::string master;

public:
    Dog(int t = 1, std::string n = "", int a = 0, std::string b = "", std::string s = ""):
        Animal(t, n, a), disease(b), master(s) {}

    void print(std::ostream& os) const override
    {
        Animal::print(os);
        os << "disease: " << disease << std::endl;
        os << "master: " << master << std::endl;
    }
};

template<typename T>
void add(std::vector<std::shared_ptr<Animal>> &vec, const T &a)
{
    auto newptr = std::make_shared<T>(a);
    vec.push_back(newptr);
}

int main()
{
    std::vector<std::shared_ptr<Animal>> Animals;

    Dog d(1,"Rex", 12, "idk", "Oscar");
    Cat c(0,"Meaw", 11, "Sfinx", "Catfood");
    add(Animals, d);
    add(Animals, c);

    /* alternatively:
    Animals.push_back(std::make_shared<Dog>(1,"Rex", 12, "idk", "Oscar"));
    Animals.push_back(std::make_shared<Cat>(0,"Meaw", 11, "Sfinx", "Catfood"));
    */

    for(auto &i: Animals)
    {
        std::cout << *i;
    }

    return 0;
}

Output:

Name: Rex
Age: 12
disease: idk
master: Oscar
Name: Meaw
Age: 11
race: Sfinx
Fav food: Catfood

Live Demo

I want to create a shared ptr vector of different objects

1 Answers1