问使用基类指针访问派生类(前向声明)

EN

   class base{
        public:
            virtual void foo()=0;
    };
    
    class derived : public base{
        public:
           int abc; // how to access this from Gtest_main.cpp
            void foo(){
                std::cout<<"This is derived foo() \n";
            }
    }; 

//Unit test framework call on createPlugin to get instance of derived class
  void createPlugin(base **plugin){
    derived *dp = new derived;
    *plugin=dp;
 }

 class derived //forward declaration 
 base *p;
 createPlugin(&p);
 static_cast<derived*>(p)->abc=9090; //error how to access abc?