Why can't we declare a std::vector<AbstractClass>?

Having spent quite some time developping in C#, I noticed that if you declare an abstract class for the purpose of using it as an interface you cannot instantiate a vector of this abstract class to store instances of the children classes.

#pragma once
#include <iostream>
#include <vector>

using namespace std;

class IFunnyInterface
{
public:
    virtual void IamFunny()  = 0;
};

class FunnyImpl: IFunnyInterface
{
public:
    virtual void IamFunny()
    {
        cout << "<INSERT JOKE HERE>";
    }
};

class FunnyContainer
{
private:
    std::vector <IFunnyInterface> funnyItems;
};

The line declaring the vector of abstract class causes this error in MS VS2005:

error C2259: 'IFunnyInterface' : cannot instantiate abstract class

I see an obvious workaround, which is to replace IFunnyInterface with the following:

class IFunnyInterface
{
public:
    virtual void IamFunny()
    {
        throw new std::exception("not implemented");
    }
};

Is this an acceptable workaround C++ wise ? If not, is there any third party library like boost which could help me to get around this ?

Thank you for reading this !

Anthony

Solution 1:

You can't instantiate abstract classes, thus a vector of abstract classes can't work.

You can however use a vector of pointers to abstract classes:

std::vector<IFunnyInterface*> ifVec;

This also allows you to actually use polymorphic behaviour - even if the class wasn't abstract, storing by value would lead to the problem of object slicing.

Solution 2:

You can't create a vector of an abstract class type because you cannot create instances of an abstract class, and C++ Standard Library containers like std::vector store values (i.e. instances). If you want to do this, you will have to create a vector of pointers to the abstract class type.

Your workround would not work because virtual functions (which is why you want the abstract class in the first place) only work when called through pointers or references. You cannot create vectors of references either, so this is a second reason why you must use a vector of pointers.

You should realise that C++ and C# have very little in common. If you are intending to learn C++, you should think of it as starting from scratch, and read a good dedicated C++ tutorial such as Accelerated C++ by Koenig and Moo.

Solution 3:

In this case we can't use even this code:

std::vector <IFunnyInterface*> funnyItems;

or

std::vector <std::tr1::shared_ptr<IFunnyInterface> > funnyItems;

Because there is no IS A relationship between FunnyImpl and IFunnyInterface and there is no implicit convertion between FUnnyImpl and IFunnyInterface because of private inheritance.

You should update your code as follows:

class IFunnyInterface
{
public:
    virtual void IamFunny()  = 0;
};

class FunnyImpl: public IFunnyInterface
{
public:
    virtual void IamFunny()
    {
        cout << "<INSERT JOKE HERE>";
    }
};