Using a Static Class function to create a Singleton object/instance

Question

I am trying to create a static member function that returns a pointer to one instance of the class. Is this possible in C++?

    class DynamicMemoryLog
{
    // Singleton Class:

    public:

        static DynamicMemoryLog* CreateLog();
        void   AddIObject( IUnknown* obj );
        void   ReleaseDynamicMemory();

    private:
        // static DynamicMemoryLog* instance;
        static bool isAlive;  // used to determine is an instance of DynamicMemoryLog already exists

        DynamicMemoryLog();
        ~DynamicMemoryLog();

        std::vector <IUnknown*> iObjectList;
};

This function below should create a new instance of the class & return a pointer to that object, but the compiler will not allow me to define a static function of the class if it returns a pointer(I think thats why it wont compile?):

static DynamicMemoryLog* DynamicMemoryLog :: CreateLog()
{
    // Post:

    if ( !isAlive )   // ( instance == NULL; )
    {
       DynamicMemoryLog* instance  = new DynamicMemoryLog();
       return instance;
    }

    return NULL;
}

DynamicMemoryLog.cpp(17): error C2724: 'DynamicMemoryLog::CreateLog' : 'static' should not be used on member functions defined at file scop — Sascha
– Sascha, Commented Jan 18, 2011 at 10:36
just a reminder, if you have more than one singleton class linked or depend each other, logging classes should be removed or released as last during program exit. Therefore, please keep in mind "object life time" management. It is usually skipped by most developers. — baris.aydinoz
– baris.aydinoz, Commented Jan 18, 2011 at 12:26

templatetypedef · Accepted Answer · 2011-01-18 10:39:16Z

2

The particular error you're getting is that when implementing a static member function, you don't repeat the static keyword. Fixing this should resolve the error.

Independently, there's something a bit odd with your code. You claim that this object is a singleton, but each call to CreateLog will create a new instance of the class. Do you really want this behavior, or do you want there to be many copies? I'd suggest looking into this before proceeding.

answered Jan 18, 2011 at 10:39

templatetypedef

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1 Comment

Sascha Over a year ago

Ah, so I dont put static at the begininng when implementing the static function. Thx :)

Bojan Komazec · Accepted Answer · 2011-01-18 10:44:11Z

1

Here's the simplest solution, but not thread-safe. For analysis in detail, have a look at this article.

class DynamicMemoryLog 
{
public:
   static DynamicMemoryLog* GetInstance();
private:
   DynamicMemoryLog();
   static DynamicMemoryLog* m_pInstance;
}

DynamicMemoryLog* DynamicMemoryLog::GetInstance()
{
   if(!m_pInstance)    
   {
      m_pInstance = new DynamicMemoryLog();       
   }     

   return m_pInstance;
}

edited Jan 18, 2011 at 10:44

answered Jan 18, 2011 at 10:35

Bojan Komazec

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3 Comments

Sascha Over a year ago

If GetInstance() is not static, I wont be able to do this will I... DynamicMemoryLog *dm = DynamicMemoryLog :: GetInstance(); ?

Bojan Komazec Over a year ago

@Sascha Static member methods can access only static class' attributes

Bojan Komazec Over a year ago

@Sascha You would usually access other members through DynamicMemoryLog::GetInstance(), e.g. DynamicMemoryLog::GetInstance()->AddIObject(...)

mtvec · Accepted Answer · 2011-01-18 10:39:41Z

0

I usually do something like this:

class Singleton
{
    public:

        static Singleton* get()
        {
            static Singleton instance;
            return &instance;
        }
};

This way you won't have any nasty memory management issues.

answered Jan 18, 2011 at 10:39

mtvec

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2 Comments

Sascha Over a year ago

Will that variable, instance, will be persistant throughout & fall out of scope?

mtvec Over a year ago

@Sascha: what do you mean with persistent and fall out of scope? The variable instance will be constructed when get() is called for the first time and destructed on program exit (or on dynamic library unload when the class lives in a library).

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