Re: [Boost-users] Best way to create a class using the singletonpattern and shared pointers?

17 May 2007

      On Thu, 2007-05-17 at 11:16 +0200, Hughes, James wrote:
...
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-----Original Message-----
From: boost-users-bounces@lists.boost.org 
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of 
Stephen Torri
Sent: 16 May 2007 23:52
To: boost-users
Subject: [Boost-users] Best way to create a class using the 
singletonpattern and shared pointers?
Here is a function that I am trying to understand why its 
giving me a error via valgrind of "Invalid read of size 4".
16:    Trace_State::ptr_t Trace_State::Instance()
17:    {
18:        // Lock the resource?
19:        if ( m_instance.get() == 0 )
20:            {
21:                m_instance = Trace_State::ptr_t ( new
Trace_State() );
22:            }
23:        
24:        // Unlock the resource?
25:        
26:        return m_instance;
27:    }
Sorry, I don't have an answer to your specific question, but why would
you need to use a shared_ptr on a singleton anyway? You have no real
need to deallocate a singleton instance, so no need for a
shared_ptr....perhaps use a reference (See Meyers singletons)
Oh, hold on a sec....is it valid to call get() on an unassigned
shared_ptr, which you are doing in the instance function?
James
Back when I first learned about the singleton pattern I used a standard
pointer similar to this example:

  class Singleton 
  {
  public:
      static Singleton* Instance();
  protected:
      Singleton();
      Singleton(const Singleton&);
      Singleton& operator= (const Singleton&);
  private:
      static Singleton* pinstance;
  };

Now I am trying to be a good programmer and keep track of my pointers.
So I decided that I should replace the pointer with a shared_ptr.

In regards to the validity of calling get() on an unassigned shared_ptr
I thought that if a shared_ptr was declared in the header and no call is
given to any constructor that the default constructor would be used.
Therefore the shared_ptr would have a value of 0x0.

Stephen