c++ - inheritance and polymorphism for class and its member at the same time (shared_ptr, C++11) -

i playing around following structure of classes use std::shared_ptr (c++11):

#include <string> #include <iostream> #include <vector> #include <memory>  //class member given 3rd party library, can't modify structure! class member {//no instance of class allowed    public:      member() {};//should never called      virtual ~member() = 0;//pure virtual distructor;      virtual void foo() {        std::cout<<"member"<<std::endl;      } }; member::~member() {} //need define destructor because of child classes   class childmember : public member {   public:      childmember() {};      virtual void foo() {          std::cout<<"child member"<<std::endl;      }       virtual void foo2() {         std::cout<<"unique foo in child"<<std::endl;      } };  class base {    public:      base() {};     virtual    std::shared_ptr< member >    get_var() {return var;}     virtual void     set_var ( std::shared_ptr< member > v) { var = v;}     std::shared_ptr< member > var; };   class childbase : public base {   public:      childbase() {         //var = std::make_shared<childmember>();      };    virtual   std::shared_ptr< childmember >   get_var() {return var;} //(2) : try comment    virtual void    set_var ( std::shared_ptr< childmember > v) { var = v;}    std::shared_ptr< childmember > var; };  void func(std::shared_ptr<base> b) {     b->get_var()->foo();// process input using polymorphism }  int main()  {     std::shared_ptr<childbase> cb( std::make_shared<childbase>());     cb->set_var (std::make_shared<childmember>() );     cb->get_var()->foo2();//(3) want use unique functions of childbase;     cb->var->foo2();      //can access directly well;     //cb->var = std::make_shared<childmember>();     func(cb); } 

what try design 2 classes (base , childbase) each 1 has own member member / childmember. after full usage of childbase object , member object of type childmember, pass function func receives base , should, using polymorphism call proper foo of member var.

problems: (1) in variant in above, compiler complains that:

 overriding 'virtual std::shared_ptr<member> base::get_var()'      get_var() {return var;}` 

(2) if comment-out childbase implementation of get_var, function treated base , returns pointer member not have foo2.

(3) can comment out cb->get_var()->foo2() . whole things compiles, seems not call childmember::foo() there no output child member;

something (hopefully) messed don't see what. guys please correct it?

edit1:

based on abhijit kadam answer, following code compiles:

#include <string> #include <iostream> #include <vector> #include <memory>  class member {//no instance of class allowed    public:      member() {};      virtual ~member() = 0;//pure virtual distructor;      virtual void foo() {        std::cout<<"member"<<std::endl;      } }; member::~member() {} //need define destructor child classes   class childmember : public member {   public:      childmember() {};      virtual void foo() {         std::cout<<"child member"<<std::endl;      }       void foo2() {         std::cout<<"unique foo in child"<<std::endl;      } };  class base {    public:      base() {};       std::shared_ptr< member >      get_var() {return var;}       void set_var ( std::shared_ptr< member > v) { var = v;}       std::shared_ptr< member > var; };   class childbase : public base {   public:      childbase() {        //var = std::make_shared<childmember>();      };         std::shared_ptr< childmember > var; };  void func(std::shared_ptr<base> b) {     b->get_var()->foo();// process input using polymorphism }   void func_vec( std::vector<                std::shared_ptr<base>            > vec) {      (unsigned int i=0;i<vec.size();i++)           vec[i]->get_var()->foo();   }  int main()  {    std::shared_ptr<childbase> cb( std::make_shared<childbase>());    cb->set_var (std::make_shared<childmember>() );     func(cb);     std::vector< std::shared_ptr<base>> vec;    vec.push_back(cb);    func_vec(vec);     cb->var->foo2();     /*std::shared_ptr<childmember> ptr(std::dynamic_pointer_cast<childmember>(cb->get_var()) );    if (ptr) {       ptr->foo2();    } */ } 

edit2 added answer below.

as return types of overriding functions cannot differ have use dynamic cast implement solution. hence no need have virtual functions `get_var , set_var'. no need foo2 virtual unless class further derived other class.

int main()  {     std::shared_ptr<childbase> cb( std::make_shared<childbase>());     cb->set_var (std::make_shared<childmember>() );      shared_ptr<childmember> ptr(dynamic_pointer_cast<childmember>(cb->get_var()) );     if (ptr) {         ptr->foo2();     }  }  class member {//no instance of class allowed    public:      member() {};      virtual ~member() = 0;//pure virtual distructor;      virtual void foo() {        std::cout<<"member"<<std::endl;      } }; member::~member() {} //need define destructor child classes   class childmember : public member {   public:      childmember() {};      virtual void foo() {          std::cout<<"child member"<<std::endl;      }       void foo2() {         std::cout<<"unique foo in child"<<std::endl;      } };  class base {    public:      base() {};     std::shared_ptr< member >    get_var() {return var;}     void set_var ( std::shared_ptr< member > v) { var = v;}     std::shared_ptr< member > var; };   class childbase : public base {   public:      childbase() {         //var = std::make_shared<childmember>();      };      std::shared_ptr< childmember > var; };  void func(std::shared_ptr<base> b) {     b->get_var()->foo();// process input using polymorphism }