How can i call a derived class method using a base class object ?

[claus1911]

I understand how this goes in the opposite direction. But for various reasons i want to use a base class object to call a derived class method

Let's say we have 2 classes, together representing a person's data (x= age and y=height) :

    class Person{

protected:
    int x;

public:

    Person()
    {
        x=0;
    }

    Person(int x1)
    {
        x=x1;
    }

    Person(const Person &o1)
    {
        x=o1.x;
    }

    Person &operator=(const Person &o1)
    {
      x=o1.x;
      return *this;
    }

    int get_x() const
    {
        return x;
    }

    virtual float get_y() const = 0;

    virtual ~Person() = default;

};


class Info: public Person{

protected:
    float y=10;

public:

    Info(): Person()
    {
        y=0.0;
    }

    Info(int x1, float y1): Person(x1)
    {
        y=y1;
    }

    Info(const Info &o2): Person(o2)
    {
        y=o2.y;
    }

    Info &operator=(const Info &o2)
    {
        y=o2.y;
        Person::operator=(o2);
        return *this;
    }

    float get_y() const override
    {
        return y;
    }
};

Since those 2 classes are about a person's data, and i have a Person object, i want to use this object to find out his height as well ( likely calling the method get_y() from it's derived class, Info)

From what i've readed, it has to do something with virtual methods ( pure virtual methods). So i tried to use a pure virtual method but now the problem is that i can't call that method in main properly (im too stupid to figure it out how to):

    Person* o1;
    o1->get_y(); /// didn't work..
    return 0;

How can i do it ? (i will apreciate if you can show me the main of the program too).

[Ganado]

First, you should avoid using variable names like 'x' and 'y' (outside limited cases, like actual coordinate system functions). Why not call them what they are -- "age" and "height"?

Second, in my opinion you are constructing the wrong class hierarchy, and it's leading you down the wrong path. Inheritance is supposed to represent an "is-a" relationship. A Dog "is an" Animal. But an Info is not a Person. More properly, a Person has Info associated with him or her. This is a has-a relationship, or aggregation.

What I would do instead: Start with a Person class. Let a Person have Age and Height properties (whether you simply make these be ints/doubles, or whole classes onto themselves, is up to you). Or, make an Info class that has age and height properties.

// Example program
#include <iostream>
#include <string>

struct Info
{
    int age;
    double height;
};

class Person
{
  public:
    Person(int age, double height)
     : info {age, height} { }
    
    const Info& getInfo()
    {
        return info;   
    }
    
  private:
    Info info;
    
};

int main()
{
    Person bobby(13, 165.0);
    std::cout << bobby.getInfo().age << '\n';
}