C++ polymorphism. Methods - c++

Question

I have a small problem which I can't handle.
Currently I'm working over a project about a marathon between animals.
I'm obliged to use polymorphism even though it could be easier without.
Here's a sample of my code:
class Animal
{
public:
virtual void run()=0;
virtual bool return_if_finished()=0;
virtual float return_distance()=0;
}
class Turtle :public Animal
{
int id;
float distance; //etc.
public:
void run();
bool return_if_finished();
float return_distance();
void set_id(int i);
void a_friend();
}
class Snail :public Animal
{
float distance; //etc.
public:
void run();
bool return_if_finished();
float return_distance();
void broken_leg();
}
So that's a sample. All classes that inherit from the main class "Animal" have only three mutual methods. They also have some that only they do need.
If I want to write a code in a method where they "run" like that:
...
Animal* turtles = new Turtle[amount];
Animal* snails = new Snail[amount];
for(int i=0; i<amount; i++)
turtles[i].set_id(i);
I can't compile it because "class Animal has no member called "set_id"".
I could create all these methods for each class but that would be totally pointless. I bet there's a quick solution to that.
If I create a virtual void "set_id(int)" for the class "Animal" then I get the error message that not all classes that inherit from animal contain that method.
So any help would be very appreciated. Thank you

Answer 1

If I create a virtual void "set_id(int)" for the class "Animal" then I get the error message that not all classes that inherit from animal contain that method.
I suspect you defined Animal::set_id as a pure virtual, like this:
virtual void set_id(int) = 0;
What you really want is to define it in the Animal class as a virtual method, like this:
virtual void set_id(int _id) {id = _id};
Also, the id member variable needs to be moved to the Animal class instead of Turtle
EDIT:
Expanding the answer to include the full code:
class Animal
{
public:
Animal() : id(-1) {}
virtual ~Animal() {}
virtual void run() = 0;
virtual bool return_if_finished() = 0;
virtual float return_distance() = 0;
void set_id(int i) { id = i; }
private:
int id;
};
class Turtle :public Animal
{
public:
void run() {};
bool return_if_finished() { return true; };
float return_distance() { return 2.0; };
void a_friend() {};
};
class Snail :public Animal
{
public:
void run() {};
bool return_if_finished() { return false; };
float return_distance() { return 1.0; };
void broken_leg() {};
};
int main()
{
const int amount = 10;
Turtle turtles[amount];
Snail snails[amount];
for (int i = 0; i < amount; i++) {
turtles[i].set_id(i);
}
}

Answer 2

First of all, using:
Animal* turtles = new Turtle[amount];
Animal* snails = new Snail[amount];
is a bad idea.
The pointer arithmetic on turtles and snails will be based size of Animal. If you use tutles[i] for all i not equal to 0, you'll run into undefined behavior. There is probably an SO question somewhere about that.
Use a vector of pointers instead. It will be also easier to initialize them.
std::vector<Animal*> turtles(amount); = new Turtle[amount];
for(int i=0; i<amount; i++)
{
Turtle* tptr = new Turtle;
tptr->set_id(i);
turtles[i] = tptr;
}
Better yet, use a smart pointer.
std::vector<std::shared_ptr<Animal>> turtles(amount); = new Turtle[amount];
for(int i=0; i<amount; i++)
{
Turtle* tptr = new Turtle;
tptr->set_id(i);
turtles[i] = std::shared_ptr<Animal>(tptr);
// Or
// turtles[i].reset(tptr);
}