Example of polymorphism working with data structure in C++ based on php example

Question

I am learning polymorphism and I am familiar with php.

I came across this excellent example from https://stackoverflow.com/a/749738/80353. reproduced below.

How

Answer 1

As someone not familiar with C++, create a index.cpp file and fill it up with the following

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

using namespace std;

class Animal {
  public:
    std::string name;
    Animal (const std::string& givenName) : name(givenName) {}
    virtual string speak () = 0;
    virtual ~Animal() {}

  };

class Dog: public Animal {
  public:
    Dog (const std::string& givenName) : Animal (givenName) {

    }
    string speak ()
      { return "Woof, woof!"; }
  };

class Cat: public Animal {
  public:
    Cat (const std::string& givenName) : Animal (givenName) {
    }
    string speak ()
      { return "Meow..."; }
  };

int main() {
    std::vector<std::unique_ptr<Animal>> animals;
    animals.push_back( std::unique_ptr<Animal>(new Dog("Skip"))  );
    animals.push_back( std::unique_ptr<Animal>(new Cat("Snowball"))  );

    for( int i = 0; i< animals.size(); ++i ) {
        cout << animals[i]->name << " says: " << animals[i]->speak() << endl;
    }

}

afterwards, compile the index.cpp file with the following command

c++ index.cpp -std=c++11 -stdlib=libc++

you need that because of the use of smart pointers unique_ptr in the code.

finally you can execute the compiled executable output file which should be a.out

./a.out

Answer 2

Well, you can use a pointer with abstract classes using pure virtual methods like so:

class Polygon {
    public:
        virtual void setValue(int k) = 0;    // Declaring our pure virtual method.
};

class Rect : public Polygon {
    public:
        virtual void setValue(int k); = 0;   // Declaring the pure virtual method again.
};

Rect::setValue(int k) {    // You must create a setValue() method for every class, since we made a pure virtual method. This is the Polymorphic part.
    // Code in here that setValue() method executes.
    int foo = a;
    std::cout << foo << std::endl;
}

Now you can access the methods declaring object pointers.

int main() {
    Polygon* pSomePolygon = nullptr;   // Assuming using C++11.
    Rect* pRect = new Rect;    // Declare our object.

    pSomePolygon = pRect;    // Point our pointer to object 'pRect'.

    pSomePolygon->setValue(18);    // Pointer accesses pRect and uses the pure virtual method.
    delete pRect;    // Clean up!

    return 0;
}

Answer 3

Since you only asked about storing them, I'll omit the implementation of Animal, Dog and Cat.

vector< shared_ptr<Animal> > animals;
animals.push_back( new Dog("Skip") );
animals.push_back( new Cat("Snowball") );

for( size_t i = 0; i< animals.size(); ++i )
    cout << animals[i]->name << " says: " << animals[i]->speak() << endl;

Essentially you need to store pointers to the objects. Put simply, shared_ptr<T> is a class that stores a pointer and deletes it when it is no longer referenced.