Question

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

Answer 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.