Re: Singleton --- Just Look and give Suggestion's

anon <>
Fri, 06 Mar 2009 08:41:20 +0100
Vaclav Haisman wrote:

I suggest you do not use the singleton pattern at all.

Pallav singh wrote, On 5.3.2009 17:16:

1. its not executing display() properly ....just look to it
2. Suggest effective way to write MUTEX class at User level

#include <iostream.h>

Bad header, use <iostream>.

using namespace std;

This code will be in header, do _not_ use "using namespace foo;" in headers.
You polute the namespace for all users of your header.

template<typename TYPE>
class Singleton
  private :
    Singleton() { cout<<" constructor Invoked "<< endl; }
  ~Singleton() { cout<<" Destructor Invoked "<<endl; }
    Singleton(const Singleton<TYPE> & a)
         { cout<<" Copy Constructor Invoked "<<endl; }

   const Singleton<TYPE> & operator = (const Singleton<TYPE> &);

   static TYPE * instance;
   static volatile long Flag; // Flag is volatile.

  public :
   static TYPE * GetInstance( void )
        if( Flag == 0 )
              // TO BE DONE Guard<Mutex> acquire();
                if( Flag == 0 )
                           if( instance != NULL)
                            { try
                                   { instance = new TYPE();}
                                   { cout <<"Creation of Object failed
"<<endl; }

Let the execption propagate. Returning NULL here will only misteriously break
things elsewhere.

                     cout<<" Instance Created Successfully \n";

                     Flag = 1;
             // Mutex.release();
        return instance;
           cout<<" Returning the Already Created Instance \n";
           return instance;

template<typename TYPE>
TYPE * Singleton<TYPE>::instance = 0 ;

template<typename TYPE>
volatile long Singleton<TYPE>::Flag = 0;

class A
  public :
     int i,j;
     A(int i = 1 , int j = 1):i(i),j(i){}

     void display()
     { cout<<" value of i "<< i <<" value of j "<< j <<endl; }

int main()
   A * obj1 = Singleton<A>::GetInstance();
   A * obj2 = Singleton<A>::GetInstance();
   A * obj3 = Singleton<A>::GetInstance();


  // To check if it call destructor of Object
   delete obj1;
   delete obj2;
   delete obj3;

Your class A is not singleton in itself; I can still write "new A" anywhere
else in the code. That you use Singleton<A> does not imply/force single
instance of A.

True. There are better ways to implement singleton

Singleton is glorified memory leak. Your A's dtor will never be called, bad.

It will be called at the process end

You cannot use your singleton in other code that runs during global/static
variables initialization, if you intend to have global/static mutex to
synchronize the singleton instance creation. That limits its use.

Using template like that or anything similar also makes your singletons not
really having single instance when DLLs on Windows.

I agree - windows is shit.

Bottom line, singleton is bad idea. Properly managed life time of globally
visible pointer to A is much much better.

Then why are singleton questions appearing in this NG all the time?
It's uses are limited, but it is a very good idea

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