run time type information (why this code don't work)

Question

I have a code similar to this (simplified to help present the problem)

  class a
  {
   protected:
      int m_x;
  public: 
      a(int x):m_x(x){};
      ~a(){};
      virtual int GetX()=0;
  }

  class b:public a
  {
   public: 
   b(int x):a:(x){};
    ~b(){};
      virtual int GetX(){return m_x+2;};
  }
  class c:public a
  {
   public: 
   c(int x):a:(x){};
    ~c(){};
      virtual int GetX(){return m_x+4;};
  }

I also have these functions:

vector<a> GetData()
{
     vector<a> data;
     data.push_back(b(1));
     data.push_back(c(1));
 }

 void printData()
 { 
      vector<a> data=GetData();
      for(int I=0;i<data.size();I++)
      {
             cout<< data[I].GetX()<<endl;
      }
 }

The above program did not compile by error that a class with virtual function can not be instantiated.

so I changed a to this one:

class a
  {
   protected:
      int m_x;
  public: 
      a(int x):m_x(x){};
      ~a(){};
      virtual int GetX()={return m_x};
  }

But I am not getting the correct result as I thought since I created objects of type b and c then when I call GetX, their function should be called and not of a. so I am getting this data:

1
1

instead of

 3
 5

How can I fix this problem?

Is there any switch in compiler that I should turn on to get this to work?

I am using visual studio 2012.

Answer 1

You need to use a vector of pointers. Fortunately, since you are using VS2012, you have modern smart pointers (with some other C++11 goodness):

typedef std::vector<std::unique_ptr<a>> DataVector;

DataVector GetData() {
    DataVector data;
    data.push_back(std::unique_ptr<b>(new b(1)));
    data.push_back(std::unique_ptr<c>(new c(1)));
    return data;
}

void PrintData() { 
    for(const auto & datum : GetData())
        std::cout << datum->GetX() << std::endl;
}