[boost] Singleton Library Ready for Review

6 Feb 2005

      Recent changes:

* The dependency lifetime now works by using the pointers themselves as 
dependencies, and using lazy creation to determine whether creation of 
the pointer or dereferencing of the pointer creates the instance.  Using 
the pointer instead of a separate dependency class allows the lifetime 
to achieve a consistent interface with the other lifetime policies.

* All lifetime policies now work uniformly for Singletons, Multitons, 
and potentially any Somethingtons that an end user might choose to 
implement (HashedMultiton, TwoKeyedMultiton, etc...).  The only caveat 
is that the Multiton instance holder (a singleton used in the 
implementation of the multiton), must use a leaky lifetime.  If any 
other type of lifetime is used, combining the longevity lifetime policy 
with the multiton ends up being disastrous.  If the OS cleans up leaked 
memory when the program exits, this should not be a problem.  Even so, I 
would like to find a better solution if possible.

Because I have no experience with multi threading, I need someone to 
verify that I am locking in all of the right places, and that any 
singleton/multiton operations that need to be thread safe actually are 
thread safe.  A sample test program is provided below, whose lifetime 
policies can be easily edited to verify order of creation and 
destruction in all circumstances.

I am fairly confident that my singleton library is now complete.  What 
are the next steps to take in adding it to Boost?

-Jason

// Sample Program

#include <sstream>
#include <fstream>

#include "../Include/StaticAllocator.h"
#include "../Include/MallocAllocator.h"
#include "../Include/LongevityLifetime.h"
#include "../Include/Singleton.h"
#include "../Include/Multiton.h"

using namespace boost::singleton;
using std::stringstream;
using std::ofstream;

class TestLog : public Singleton < TestLog, LongevityLifetime < TestLog, 
10 > >
{
private:
    ofstream out;

protected:
    TestLog (  ) : out ( "log.dat" )
    {
        out << "Test log created.\n";
    }

    ~ TestLog (  )
    {
        out << "Test log destroyed.\n";
        out.close (  );
    }

public:
    void Write ( const char * msg )
    {
        out << msg;
    }
};

class A : public Multiton < int, A, LongevityLifetime < A, 5 > >
{
private:
    TestLog :: Pointer log;

protected:
    A (  )
    {
        log->Write ( "A created.\n" );
    }

    ~ A (  )
    {
        log->Write ( "A destroyed.\n" );
    }
};

// lifetime longevity higher than A, to verify that even though A is 
created first, it is still destroyed after B
class B : public Singleton < B, LongevityLifetime < B, 7 > >
{
private:
    TestLog :: Pointer log;
    A :: Pointer a1;
    A :: Pointer a2;
    A :: Pointer a3;

protected:
    B (  ) : a1 ( 0 ), a2 ( 1 ), a3 ( 1 )
    {
        log->Write ( "B created.\n" );
    }

    ~ B (  )
    {
        log->Write ( "B destroyed.\n" );
    }

public:
    void foo (  )
    {
        log->Write ( "foo called.\n" );
    }
};

int main (  )
{
    B::Pointer ptr;
    ptr->foo (  );
}

[boost] Singleton Library Ready for Review

Jason Hise