Need help with map (c++, STL)

Question

Hi folks!

Actually I'm new to C++. I tried something out (actually the map container) but it doesn't work the way I assumed it will... Before posting my code, I will explain it shortly.

I created 3 classes:

ClassA ClassDerivedA ClassAnotherDerivedA

The two last ones are derived from "ClassA".

Further I created a map:

  map<string,ClassA> test_map;

I put some objects (from Type ClassDerivedA and ClassAnotherDerivedA) into the map. Keep in mind: the mapped value is from type "ClassA". This will only work because of Polymorphism. Finally I created an iterator which runs over my map and compares the user input with my keys in the map. If they match, it will call a specific method called "printOutput".

And there is the Problem: Although i declared "printOutput" as "virtual" the only method called is the one from my base class, but why? and here is the code:

#include <iostream>
#include <map>

using namespace std;

class ClassA
{
    public:
        virtual void printOutput() { cout << "ClassA" << endl;      }
};

class ClassDerivedA : public ClassA
{
    public:
        void printOutput() { cout << "ClassDerivedA" << endl;       }
};

class ClassAnotherDerivedA: public ClassA
{
    public:
        void printOutput() { cout << "ClassAnotherDerivedA" << endl;        }
};

int main()
{
    ClassDerivedA class_derived_a;
    ClassAnotherDerivedA class_another_a;


  map<string,ClassA> test_map;
    test_map.insert(pair<string,ClassA>("deriveda", class_derived_a));
    test_map.insert(pair<string,ClassA>("anothera", class_another_a));

    string s;

    while( cin >> s )
    {
    if( s != "quit" )
    {
        map<string,ClassA>::iterator it = test_map.find(s);
      if(it != test_map.end())
        it->second.printOutput();
    }
    else
      break;
    }

}

Answer 1

C++ is not Java. You cannot store a derived type in a variable of a base type. For example:

Base b = Derived();

will only store the Base part of Derived in the variable b. In order to get polymorphic behaviour, you would need to use pointers, and create the derived class dynamically:

Base * b = new Derived();

The same goes for C++ containers - you need:

map <string, Base *> m;

All of this should be covered in every introductory C++ text book - which one are you using?

Answer 2

The problem is slicing. You are storing ClassA values in your map. When you store derived class instances into the map, the get sliced into ClassA objects. You'll need to store pointers in your map instead of values.

See this for more info on slicing: http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c

Answer 3

You are experiencing "slicing". To get the virtual functions to work properly, you need to call them using a pointer or a reference. In other words, your map should contain pointers to ClassA:

map<string, ClassA *> test_map

Please remember to delete them when you are done, or use smart pointers.

Here's more on slicing: here, here, and here