Polymorphism

Hello.
I have 3 classes. B and C inherit from A. I want that class B will be able to call functions of C, so I tried to do this:

Code:

---

class A
{
    public:
      A(int b=0) {a=b;}
    protected:
      void Print() {cout<<"Hello world"<<endl;}
    private:
      int a;
};
class B: public A
{
    public:
      B(int b=0): A(b){}
      void Do(A& a) {a.Print();} //error
};
class C: public A
{
    C(int b=0): A(b){}

    int c;
};

---

but i get an error:
error: `virtual void A::Print()' is protected.

How can I do it without using "friend" (and without making Print public) ?

Why does A not have any virtual functions, not even a virtual destructor? It looks like you cannot use inheritance based polymorphism unless you change A.

Other than that... what does B have to do with C?

Quote:

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Originally Posted by omerd

Hello.
I have 3 classes. B and C inherit from A. I want that class B will be able to call functions of C, so I tried to do this:

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In that case B and C must be related by inheritance like for example,

class A {};
class C : public A {};
class B : public C {};

Both B and C inherits A, C directly and B indirectly via C.

This is also possible,

class A {};
class C : public A {};
class B : public A, public C {};

It's essentially as before apart from that if A carries implementation it's now an example of the (in)famous so called diamond inheritance. You can resolve the ambiguity by the use of virtual inheritance like this,

class A {};
class C : virtual public A {};
class B : virtual public A, public C {};

If the purpose if inheriting C in B was only to get at the implementation of C then you can inherit C privately, like

class A {};
class C : virtual public A {};
class B : virtual public A, private C {};

Now B isn't a C class as it was before, Just an A class.

As you can see there are many options and ample opportunity to make a mess out of a design in C++. :)

Quote:

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error: `virtual void A::Print()' is protected.

---

This error shows the code you shown is incorrect. Please post the actual snippet, otherwise we can't help you too much.

Your thread title, code and description don't have much in common.

There's no polymorphism in the code you provided.

You're not trying to call in functions in C and the code you provided doesn't produce the error you report.

What are you really trying to do?

maybe
I changed the code a little bit. Now it is more specific, but I also tried to keep it simple.
Actually, B and C manage a list of objects of type D, and in some cases B need to get C's D objects. B and C create objects of D in different ways.
If it's help you, C holds a list of object of type B.

Code:

---

class D
{
    public:
      D(string s): name(s)
        {another_data =-1;}
      D(string s,int data): name(s),another_data(data)
        {}
      D(const D& d)
      {/*code*/}
      string GetName() {return name;}
      int GetData() {return another_data;}

    private:
      string name;
      int another_data;
};

class A
{
    public:
      A(int b=0) {a=b;}
      virtual ~A()
      {
          //code;
        }
    protected:
      virtual D* FindD(string)=0;
    private:
      int a;
};
class B: public A
{
    friend class A;
    public:
      B(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }
      void Do(A& a, string name)
      {
          a.FindD(name); //error: `virtual D A::FindD(std::string)' is protected|
      }

    private:
    list<D> names;

};
class C: public A
{
    C(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }

    list<D> names;
};

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Why not put the list in A?

you right, but it doesn't help. class B need to call "FindD" on objects of type C (which is the argument in function "Do")

Quote:

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Originally Posted by omerd

you right, but it doesn't help. class B need to call "FindD" on objects of type C (which is the argument in function "Do")

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Okay, so class B would need an instance of class C to do that. What's the problem? Just call FindD from your B object using your instance of C.

From an inheritance POV, if B and C both have a list of D, that list should be in a common base class.

Quote:

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Originally Posted by GCDEF

Okay, so class B would need an instance of class C to do that. What's Just call FindD from your B object using your instance of C.

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that what's I did. B call "FindD" on C's object, and I get an error.

Quote:

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Originally Posted by omerd

that what's I did. B call "FindD" on C's object, and I get an error.

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Show the smallest and simplest compilable program that demonstrates that error.

Quote:

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Originally Posted by omerd

that what's I did. B call "FindD" on C's object, and I get an error.

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Don't you think explaining the error a bit may help us help you?

Quote:

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Originally Posted by GCDEF

Don't you think explaining the error a bit may help us help you?

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I wrote the error above:
error: `virtual void A::Print()' is protected.

Quote:

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Originally Posted by laserlight

Show the smallest and simplest compilable program that demonstrates that error.

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I will edit this message in a few hours with the code.

So either make FindD public in C, or else declare that B is a friend of C. I'm not sure which is more appropriate in this specific situation.

But B and C are both derived from A. Both classes contain a list of D. Why not move both the list and the FindD function into A?

Even if list<D> and FindD are A's protected members, B can't call FindD on C's object.

Consider this:

Code:

---

...
class A
{
        friend class B;
...

---

Now class B may call protected function from class A.

But actually, there is too much useless code you provide us, that's why we are just guessing what you expect to be an answer to your question.

This is the code. i tried to keep it simple as I can. The error appear in "add_line_to_bus_station" function

Code:

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#include <iostream>
#include <string>
#include <list>
using namespace std;
class Bus_Line;
class Address
{
  public:
    Address(const string street,int HouseNum): m_street(street),m_HouseNum(HouseNum){}
    int get_house_num(){return m_HouseNum;}
    string get_street(){return m_street;}
    friend bool operator== (const Address& a1,const Address& a2);

  private:

  string  m_street;
  int m_HouseNum;
};
class Bus_Station
{
public :
  Bus_Station(const Address& address): m_address(address){}
  void add_line(int bus_sine);
  const Address& get_address()
      {
          return m_address;
      }

  private:
    list<int> m_lines;
    Address m_address;
};
class Container
{
  public:
    Container(string name): m_name(name){}
    //void add_street(string str);
    //bool add_address(string street_name,int num);
    //bool add_bus_station(const Address& add);

protected:
    string m_name;
    list<Bus_Station> m_bus_stations;
    list<Address> m_addresses;
    Bus_Station* find_bus_station(const Address& add);
};
class City : public Container
{

  public:
    City(string name): Container(name) {}

    void add_line_to_bus_station(Container*c,const Address& add,int line_number); //this function uses "find_bus_station "

  private:
    list<Bus_Line> m_lines;
};

class Country : public Container
{
  public:
  Country(string name);

  //City& add_city(string city_name);
  //City* find_city(string city_name);

  private:
  list<City> m_cities;
};
class Bus_Line
{
public :


  Bus_Line(int line_code);
  int GetLineCode() {return m_line_code;}

  private:
  int m_line_code;

};
inline Bus_Station* Container::find_bus_station(const Address& add)
{
    if(m_bus_stations.empty()) {return 0;}

    list<Bus_Station>::iterator it = m_bus_stations.begin();
    while (it != m_bus_stations.end())
    {
        const Address& temp =(*it).get_address();
        if( temp == add)
        {
            return &(*it);
        }
        it++;
    }
    return 0;
}
void City::add_line_to_bus_station(Container*c,const Address& add,int line_number)
{
    Bus_Station* A= (*c).find_bus_station(add);
    if(A==0){return;}
    (*A).add_line(line_number);
}
void Bus_Station::add_line(int Bus_Line)
{
    m_lines.push_back(Bus_Line);
}
bool operator== (const Address& a1,const Address& a2)
{
    if( a1.m_street == a2.m_street && (a1.m_HouseNum == a2.m_HouseNum))
    {return true;}

    return false;
}

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Any answer? you asked me to post my code

Quote:

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Originally Posted by omerd

Any answer? you asked me to post my code

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I think you need to take a big step backwards and try to learn more about inheritance. You have a class Container. In order for City to derive properly from Container, a City would have to be a Container, and that doesn't seem likely.

Also, since in your hierarchy a City is a Container, why are you passing a Container pointer to a City function? None of what you're doing makes a whole lot of sense conceptually.

City should be a Container. I know that the name doesn't explain the meaning, but a container is an object which contains Street(s), Address(es) and Bus_Station(s). It should be mentioned that some of the bus stations are located out of town.

The paramater of "add_line_to_bus_station" is of type Container because it can be either City or Country.

Quote:

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Originally Posted by omerd

City should be a Container. I know that the name doesn't explain the meaning, but a container is an object which contains Street(s), Address(es) and Bus_Station(s). It should be mentioned that some of the bus stations are located out of town.

The paramater of "add_line_to_bus_station" is of type Container because it can be either City or Country.

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I'm still unclear why you're passing a pointer to a Container to City::add_line_to_bus_station. Since city is a Container, why not operate on the City object directly? You won't run into the protected issue if you do, and at least on the surface it seems to make more sense.

Because Country is also a Container, so the parameter might also be a Country

Quote:

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Originally Posted by omerd

Any answer? you asked me to post my code

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Some suggestions were already given but you just couldn't notice them because
A) your mind is bent on thinking that you're right, B) you're too busy justifying the questionable logic you asked us to review, C) and at the same time trying to find fault with the possible solutions given by the members of this forum.

Sometimes thinking things backwards helps.
What I mean by this is that you set aside your efforts to understand the conceptual term polymorphism for now. Instead, try to write a virtual function used in an inheritance hierarchy. I think with this approach, you can focus your synergy on more syntactical side of the language, and solidify your understanding towards the semantic side from it.

So the next sensible thing for me to do is to ask you if you know how to write a virtual function using Foo as a name of the base class and Bar as a name of the derived class with a virtual function named Print that takes no parameter.

Can you show, in code, that you could do this?

Quote:

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Originally Posted by potatoCode

Some suggestions were already given but you just couldn't notice them because
A) your mind is bent on thinking that you're right, B) you're too busy justifying the questionable logic you asked us to review, C) and at the same time trying to find fault with the possible solutions given by the members of this forum.

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I think you're right, thanks. However, the most beneficial solution I heard so far is to make Container a friend of City, but I prefer a better solution because as far as I know (and tell if it doesn't true), I should avoid using this way.

Quote:

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Originally Posted by potatoCode

Sometimes thinking things backwards helps.
What I mean by this is that you set aside your efforts to understand the conceptual term polymorphism for now. Instead, try to write a virtual function used in an inheritance hierarchy. I think with this approach, you can focus your synergy on more syntactical side of the language, and solidify your understanding towards the semantic side from it.

So the next sensible thing for me to do is to ask you if you know how to write a virtual function using Foo as a name of the base class and Bar as a name of the derived class with a virtual function named Print that takes no parameter.

Can you show, in code, that you could do this?

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Regarding to this, you're right, and the last code I've postet doesn't use polymorphism - no virtual function and no Print function without parameters.

Quote:

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Originally Posted by omerd

Because Country is also a Container, so the parameter might also be a Country

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But a Country is also a Container, so if you're trying to find a Bus Station in a Country object, why would you use a City object to do it?

You really need to back up and get a handle on how inheritance works.

Quote:

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Originally Posted by GCDEF

But a Country is also a Container, so if you're trying to find a Bus Station in a Country object, why would you use a City object to do it?

You really need to back up and get a handle on how inheritance works.

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So.. what should I do? City manages Bus_Line list so it has to get a pointer to Bus_Stations of Container.
In your opinion, how should I build the inheritance structure?

I don't know exactly what you're trying to do but I don't see City and Country being derived from Container. It seems you only want one Container object that contains the list of bus stations and addresses. By deriving from it, every City and Country object also contains the copies of the lists, which I can't see that you want.

Quote:

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Originally Posted by GCDEF

I don't know exactly what you're trying to do but I don't see City and Country being derived from Container. It seems you only want one Container object that contains the list of bus stations and addresses. By deriving from it, every City and Country object also contains the copies of the lists, which I can't see that you want.

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No, City and also Country have to hold lists of adresses and bus stations. Concerning cities, it's clear why they should hold those lists. But some of the bus stations are located out of town, so it is necessary to manage a list of all those bus stations.
However, only city should hold a list of bus lines.

p.s. GCDEF, I'm appreciate your patience for my troubles :)