Why Can\'t I Create a `std::vector` of Abstract Classes?

Understanding the Prohibition of std::vector

In object-oriented programming, abstract classes serve as blueprints for other classes to inherit from. However, they cannot be instantiated directly due to their incomplete nature. This limitation extends to declaring a std::vector of an abstract class, raising an error that states "cannot instantiate abstract class."

The Abstract Class Issue

Abstract classes are declared with pure virtual functions, which have no implementation. This means they are not concrete entities that can be created as objects. Instead, they are meant to be inherited and their abstract functions implemented in the child classes.

Workarounds

To overcome this issue, there are two main workarounds:

1. Vector of Pointers to Abstract Classes:
   Instead of declaring a std::vector of abstract classes, you can use a std::vector of pointers to abstract classes. This allows you to store instances of child classes while preserving polymorphic behavior.

   std::vector<IFunnyInterface*> ifVec;

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2. Derived Class Vector:
   Instead of using an abstract class, you can create a derived class that inherits from the abstract class and provides concrete implementations of all its virtual functions. You can then declare a std::vector of this derived class.

   class FunnyDerived : public IFunnyInterface
   {
       // Implement abstract function
   };
   
   std::vector<FunnyDerived> fdVec;

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Conclusion

While it may seem counterintuitive to restrict the creation of std::vectors of abstract classes, it is a fundamental aspect of object-oriented programming. By embracing the use of pointers to abstract classes or derived classes, you can maintain flexibility and polymorphism while adhering to the limitations of abstract classes.

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