Why can't we declare a std::vector?

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 #include using namespace std; class IFunnyInterface { public: virtual void IamFunny() = 0; }; class FunnyImpl: IFunnyInterface { public: virtual void IamFunny() { cout << ""; } }; class FunnyContainer { private: std::vector 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 li…

Accepted Answer

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.

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

Solution

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