Re: StateFull vs Stateless Singleton

From:
"Leigh Johnston" <leigh@i42.co.uk>
Newsgroups:
comp.lang.c++
Date:
Mon, 5 Jul 2010 13:09:04 +0100
Message-ID:
<_rOdnRT9maHAVqzRnZ2dnUVZ8sSdnZ2d@giganews.com>
"sebastian" <sebastiangarth@gmail.com> wrote in message
news:8de2ad05-a5a2-4bf2-a5df-f28f91ed55bf@i28g2000yqa.googlegroups.com...

On Jul 4, 6:31 pm, ?? Tiib <oot...@hot.ee> wrote:

On 4 juuli, 23:58, sebastian <sebastianga...@gmail.com> wrote:

On Jul 4, 9:02 am, Pallav singh <singh.pal...@gmail.com> wrote:

Hi

what the consideration parameter to choose stateless and statefull
Singleton ?

Thanks
Pallav Singh


Neither - a simple template class eliminates the issue altogether:

<code>

#include <stdexcept>

template < typename Type >
class singleton
{
        public:

        singleton( void )
        {
                Type* ptr = static_cast< Type* >( this );
                if( self )
                {
                /*
                        Or what have you...
                */
                        throw std::runtime_error
                        (
                                "Error: attempt to instantiate multiple
instances of a singleton"
                        );
                }
                self = ptr;
        }

        static inline Type& instance( void )
        {
                return *self;
        }

        static inline bool exists( void )
        {
                return self != 0;
        }

        virtual ~singleton( void )
        {
                self = 0;
        }

        private:

        singleton( singleton const& );

        singleton& operator = ( singleton const& );

        static Type* self;

};

template < typename Type >
Type* singleton< Type >::self = 0;

// Example:

#include <iostream>

class foo : public singleton< foo >
{
        public:

        void bar( void ) const
        {
                std::cout << "foo::bar( )" << std::endl;
        }

};

int main( void )
{
        try
        {
                foo a;
                foo::instance( ).bar( );
                foo c; // Whoops!
                foo::instance( ).bar( );
        }
        catch( std::exception const& error )
        {
                std::cerr << error.what( ) << std::endl;
        }

}

</code>

The beauty of the design is that it allows the user to decide how to
allocate and initialize the derived class; it's sole purpose is to
make sure that multiple instances aren't created. Best of all, it's
completely generic!

Cheers.


Looks as beautiful like any other reinvented square wheel. The
requirements seem to be that such code must work:

 int main( void )
 {
     {
         foo a;
         foo::instance( ).bar( );
     }
     foo c; // Second singleton and *NO* Whoops
     foo::instance( ).bar( );
 }

Not sure why someone needs to have global state? Typical example of
singletons is a logger. Huh? So ... OK. What if i need to have
separate logs from different modules? I will then need new class for
each log to have separate singleton logger for each module? Nonsense.

What if i need to have separate logs from different threads? Then that
Logger::instance() should give me "different" singletons based on
thread ID? Huh? Why it can't just be debugging::log() and where it
logs is its internal business, no global state no nothing?

As for your code ... same thing with 10-12 lines instead of 70-80:

 #include <iostream>
 namespace foo
 {
     void bar()
     {
         std::cout << "foo::bar()" << std::endl;
     }
 }

 int main()
 {
     foo::bar();
 }

Things that contain nothing are not worth allocating or initializing.


So...your point is that you hate the singleton pattern? Got it. I hate
it when people post snarky comments for no apparent reason,
personally. To each his (and her) own, I guess.

Cheers.


Singleton is an anti-pattern as well as a pattern. Creating a fancy
template to make implementing a singleton easier is a bad idea (TM) as it
leads to more abuse of the anti-pattern case especially by newbies. The
whole chapter devoted singletons in "Modern C++ Design" by Andrei
Alexandrescu is ill-conceived for the same reason.

/Leigh

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