Override derived template class method in C++11/14

Question

I need to override the method createT() of a Base template class with the one in a Derived template class with a different number of input parameters. I've developed this very simple example:

#include <iostream>

class Helper
{
public:
  Helper(std::string s): _s(s)
  {}

  void display()
  {
     std::cout << _s << std::endl;
  }

private:
  std::string _s;
};

class Helper2
{
public:
  Helper2(std::string s, int i):
    _s(s), _i(i)
  {}

  void display()
  {
    std::cout << _s << " + " << std::to_string(_i) << std::endl;
  }

private:
  std::string _s;
  int _i;
};

template<class T> class Base
{
public:
  Base()
  {
    _pTBase = createT();
  }

  void test()
  {
    _pTBase->display();
  }

protected:
  /// Subclasses can override this method.
  virtual T* createT()
  {
    return new T("### BASE ###");
  }

private:
 T* _pTBase;

}; //template<class T> class Base


template<class T> class Derived : public Base<T>
{
public:
  Derived() : Base<T>()
  {
    _pTDerived = createT();
  }

  void test()
  {
    _pTDerived->display();
  }

protected:
  virtual T* createT()
  {
    return new T("### DERIVED ###", 5);
  }

private:

 T* _pTDerived;

}; //template<class T> class Derived : public Base<T>


int main()
{
  Derived<Helper2> a;

  a.test();

  return 0;
}

When I try to compile I receive this message:

error: no matching function for call to ‘Helper2::Helper2(const char [13])’
     return new T("### BASE ###");
                                ^

It seems that the compiler cannot use the createT() method in the derived class.

Of course everything works correctly if I use the Base class with the Helper (not the Helper2) class.

What's the right way to solve this situation?

Yes. it is the same problem. You cannot call virtual functions pointing to derived classes that has not been constructed yet. — balki, Oct 27 '18 at 02:58

score 2 · Answer 1 · answered Oct 27 '18 at 02:22

2

When a is constructed, first the Base<Helper2> base object is constructed. That constructor will call Base::createT, which requires Helper2 to have a constructor that takes one parameter. Since that doesn't exist, you get the compilation error.

When you construct a Derived<Helper> object, you get two Helper objects created: one by Base's constructor, stored in _pTBase, and one by Derived<Helper>'s constructor, stored in _pTDerived.

One way to fix this is to have the derived class call a (non-virtual) Create function, then pass the returned pointer to the base class constructor.

answered Oct 27 '18 at 02:22

1201ProgramAlarm

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Thank you for your reply. The problem is that the **Base** class is part of an external library that I cannot modify and overriding the method **Base::createT** is the only way to use it. – MarioG Oct 27 '18 at 09:08
If this is the case, then its unlikely they are expecting you to overide a virtual function that they call in the base constructor. What is the external library? – rmawatson Oct 27 '18 at 12:28
Tha external library is POCO. The problem was also shown by @chris [here](https://stackoverflow.com/questions/51872709/poco-custom-socketacceptor) but he hasn't received a response yet. – MarioG Oct 27 '18 at 21:36

Override derived template class method in C++11/14

1 Answers1