How can I use a non-trivial constructor with as little code duplication as possible?

From: William Payne (mikas493_no_spam_at_student.liu.se)
Date: 10/11/04 

Date: Mon, 11 Oct 2004 23:22:04 +0200

Hello, consider the following two classes (parent and child):

#ifndef SINGLETON_HPP
#define SINGLETON_HPP

#include <cstddef> /* NULL */

template <typename T>
class Singleton
{
public:
    static T* get_instance()
    {
       if(!t)
       {
          t = new T;
       }

       return t;
    }

private:
    Singleton()
    {
       ; /* Never reached. */
    }

    static T* t;
};

class int_singleton : public Singleton<int>
{
};

/* As with member functions of class templates, the definition must *
  * be in the class header. When compilers properly support the export *
  * keyword, definitions of member functions and static data members *
  * can be moved to an implementation (.cpp) file. */
template <typename T>
T* Singleton<T>::t = NULL;

#endif /* #ifndef SINGLETON_HPP */

The int_singleton class is just an example, I will be using other
(user-defined classes) types in the "real application" (if I can make it
work).

If you look at Singleton::get_instance(), you see that when !t is true
it creates a new object of type T using the constructor that takes no
arguments. However, I need to use constructors that take several
arguments. How should I accomplish this so I get as little code
duplication as possible? I have several rather complex classes in my
program that, and there must be only one instance of each class at all
times, and I would like to use the base class Singleton as much as
possible. Maybe get_instance() should call a virtual create_object()
function when !t is true, a function I redefine in my subclasses?

Comments please, how should I solve this?

/ WP

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