Thread: Extending Classes without Virtual Methods

Originally Posted by bonechilla

If a user wanted to extend a class from someone elses library and override its methods that do not contain virtual methods; how would one call the overridden class if it is referred to by its super type

in other words how would someone override a method from someone elses library that does not have virtual keywords.

If the class has no virtual member functions (i.e., methods), then it is not intended to be a polymorphic base class, at least not via the runtime polymorphism enabled by the use of inheritance with virtual functions. Thus, by design, you would not derive from it in order to override any methods since there are no methods to override. Any attempt to override would actually result in overloading instead.

EDIT:

Originally Posted by whiteflags

Overriding a method does not require that the base class use the virtual keyword.

This is false by definition:
overriding - declaring a function in a derived class with the same name and a matching type as a virtual function in a base class. The argument types must match exactly. The return types must match exactly or be co-variant. The overriding function will be invoked when the virtual function is called.

If the base class's function isn't virtual, then it's just not possible. You would have to make your own "wrapper" which contains the necessary virtual functions which you could inherit from.

Also, do not use printf in C++. There is a I/O out object. It's called std::cout.

>>vector<character*> vec;
>> vec.push_back(new protagonist);
>> vec.push_back(new antagonist);
I suggest you change that to
vector<std::unique_ptr<character>> vec;
vec.emplace_back(new protagonist);
vec.emplace_back(new antagonist);

Code:

      for(vector<Character*>::iterator itr = vec.begin(),
                      end = vec.end();
                      itr!=end;
                      ++itr){
        std::printf("%s", (*itr)->getName());
        delete (*itr);
        cout << endl << endl;
    }

Code:

      for(auto Character : vec)
        std::cout << Character->getName()) << "\n";

>>vec.clear();
Not necessary. Redundant.

Originally Posted by laserlight

If the class has no virtual member functions (i.e., methods), then it is not intended to be a polymorphic base class, at least not via the runtime polymorphism enabled by the use of inheritance with virtual functions. Thus, by design, you would not derive from it in order to override any methods since there are no methods to override. Any attempt to override would actually result in overloading instead.

Thank you for the quick reply i figured but was not sure exactly what was happening.

Originally Posted by Elysia

If the base class's function isn't virtual, then it's just not possible. You would have to make your own "wrapper" which contains the necessary virtual functions which you could inherit from.

Code:

      for(vector<Character*>::iterator itr = vec.begin(),
                      end = vec.end();
                      itr!=end;
                      ++itr){
        std::printf("%s", (*itr)->getName());
        delete (*itr);
        cout << endl << endl;
    }

Code:

      for(auto Character : vec)
        std::cout << Character->getName()) << "\n";

Thank you Elysia i was unaware of the unique_ptr and was under the assumption that C++ did not have a for each loop.