Re: destructor is not getting called for singleton

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 29 Apr 2011 17:23:34 CST

Message-ID:

<ipdjjd$f0d$1@dont-email.me>

On 2011-04-29 04:36, Harry wrote:

Singleton.cpp
--------------
template<class T>
T * Singleton<T>::_instance = 0;

template<class T>
Singleton<T>::Singleton() {}

template<class T>
Singleton<T>::~Singleton()
{
    std::cout<<"Singlton Destructor Called"<<std::endl;
      delete _instance;
}

template<class T>
T& Singleton<T>::instance()
{
      if (!_instance)
    {
        std::cout<<"New object only created once"<<std::endl;
          _instance = new T;
    }
      return *_instance;
}

main.cpp
--------
#include "A.cpp"
int main()
{
    single::instance().test();
    return 0;
}

[..]

output:
New object only created once
Constructor A called
Testing
--

Hi gurus why the destructor is not getting called for singleton?

Because you have no object of static storage or automatic duration that
could be destroyed your. Your program just implements the moral
equivalent of:

static A* pa;

int main() {
   pa = new A();
}

and you are expecting that somehow delete will automagically be invoked
on pa.

This was a drastic reduction but it may give you a "pointer" here: Just
replace your static data member by a smart pointer, e.g.

template <class T>
std::auto_ptr<T> Singleton<T>::_instance;

and remove the explicit call of delete in the destructor of Singleton
(The smart pointer will automatically do that for you) and replace the
inner part of the static instance function by

     if (!_instance.get())
     {
    std::cout<<"New object only created once"<<std::endl;
         _instance.reset(new T);
     }

HTH & Greetings from Bremen,

Daniel Kr?gler

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