通过基本 class 指针获取派生 class object 的引用

Question

I have a vector of base class pointers to store different derived classes. I need to access methods/objects that may be present on a derived class, but not on the base class.

I've tried using a function in the form of something like base* getObject(int idx); and then using dynamic_cast , and static_cast from Base* into Derived& , but that doesn't seem to work, as compilation failed.

My base class looks something like this:

        class Base {
          int a, b;
        public:
          virtual void f1(int) = 0;
          virtual void f2(int, int) = 0;
        };

And my derived classes look something like this:

        class Derived : public Base {
          std::vector <int> myVector;
        public:
          void f1(int ai) override {}
          void f2(int ai, int bi) override {}
          int getFromVector(int idx) { return myVector[idx]; }
        }

The vector that should contain the derived classes looks like this:

        std::vector <Base*> derivedObjects;

What I would like to do, is to get something from the derivedObjects vector, and access, say, getFromVector() .

Answer 1

I would agree with Igor Tandetnik that the interface class may need to be designed better. With the given statement, dynamic_cast can be used, but you must remember that a dynamic cast is a try, which may fail. Since you are using a reference of derived class in the case of cast operation failure, you will have to catch the exception and proceed accordingly.

For example, if you have Derived1 and Derived2 classes, and you are trying to dynamically cast a Derived1 object to a Derived2 reference, then a bad_cast exception will be thrown. This happens because a reference object cannot be set to null. As an alternative, you can try to cast it to a pointer of derived type and not a reference. After trying to dynamically cast, you can check if the pointer is null.

Further reference: http://www.cplusplus.com/doc/tutorial/typecasting/