Re-usable singleton class that can construct pointers to objects with non-trivial constructors

 Eric Lilja <>
Tue, 14 Aug 2007 10:44:44 -0700
As the topic says, I wanted to make a re-usable singleton class that
could create pointers to objects with non-trivial constructors. I came
up with this:


template<typename T>
struct DefaultCreatorFunctor
   T * operator()() const { return new T; }

template<typename T>
struct ValueCreatorFunctor
   ValueCreatorFunctor(const T& val) : val_(val) {}

   T * operator()() const { return new T(val_); }

   T val_;

/* Users can add more functors for more elaborate types. */

template <typename T1, typename T2>
class Singleton
   static T1 * get_instance(const T2& creator)
      if (!obj)
         obj = creator();

      return obj;

   Singleton() {}

   static T1 *obj;

template <typename T1, typename T2>
T1 * Singleton<T1, T2>::obj = 0;

#endif /* #ifndef SINGLETON_HPP */

As you can see I've provided to default functor, one for default-
constructing and one for objects with a constructor that takes a
single value.

I've made the following test program:

#include <iostream>
#include <string>

#include "singleton.hpp"

using namespace std;

   int *n = Singleton<int, ValueCreatorFunctor<int>


   cout << *n << endl;

   delete n;

   string *s = Singleton<string, DefaultCreatorFunctor<string>


   cout << *s << endl;

   delete s;

   return 0;

it seems to work but somehow I don't feel very satisfied. The syntax
for obtaining an instance is not elegant and if you want to a more
elaborate class to be handled as a singleton, you will have to write
your own functor (basically a ValueCreatorFunctor with additional
parameters). It was however a good exercise in templates and functors
for me and for that I'm glad.

Also I was thinking about ways in which I could make the singleton
delete its pointer when the program exits, so the user doesn't have to
worry about that (and risk double deletes).

As always, I would like comments from you. :-)

- Eric

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