Re: Class templates and singleton container

From:
Francesco <xtrigger303@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 4 Nov 2008 02:39:04 -0800 (PST)
Message-ID:
<805aba16-048e-4c70-8db8-6b39a3f1f990@c2g2000pra.googlegroups.com>
(2b|!2b)==? ha scritto:

Francesco wrote:

Bit Byter ha scritto:

I want to write a (singleton) container for instances of my class
templates, however, I am not too sure on how to:

1). Store the instances
2). How to write the acccesor method (instance()) to retrieve an
instance of particular template
3). What type to return an instance as ..

Assuming I have the following code:

class template

template <class T1, class T2>
class MyTree
{
    T1 foo(const T2& a1);
    T2 foobar(const T1& a1, const T2& a2);
};

// Notes
// Instance prototype not completed (see question 2 and 3)
// Ideally, when an instance of a particular class is requested, if it
dosen't yet exist, then an
// instance is created and stored in the 'repository'
class Container
{
    instance();
};

//Main.cpp

int main(int argc, char* argv[])
{
     Container c;

     MyTree<double, int> * t1 = c.instance(/*some args here*/);
     MyTree<string, double> *t2 = c.instance(/*some args here*/);

}

Last but not the least, I want to be able to treat objects returned by
the instance() method, in a generic way (i.e. in this example, I want
to be able to treat them generically, as trees). Should I use
inheritance (i.e. the class template inherits from a base Tree class)
like this:

template <class T1, class T2>
class MyTree : public Tree
{
    T1 foo(const T2& a1);
    T2 foobar(const T1& a1, const T2& a2);
};

or is there a better way?


Hi,
I don't know exactly what you want to do, so there might be better
solutions...
But they way you've put it, I guess using something like boost::any
should do what
you want. Check the code below.
Hope it helps a little.
Bye,
Francesco

#include <vector>

#include <boost/any.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>

#include <iostream>

using namespace boost;

//------------------------------------------------------------

class CObjBase
{
public:

virtual ~CObjBase() {}

    virtual any DoSome( any const & ) = 0;

    virtual any DoOther( any const & inArg1,
            any const & inArg2 ) = 0;

    virtual bool IsOfType( std::type_info const *,
            std::type_info const * ) = 0;
};
//------------------------------------------------------------

template< typename T1, typename T2 >
class CObjConcrete : public CObjBase
{
public:

    any DoSome( any const & inArg1 )
    {
        T2 obj = any_cast< T2 >( inArg1 ); // use object
        std::cout << "-------\n";
        std::cout << "Arg1: " << obj << std::endl;
        return T1(); // return whatever;
    }

    any DoOther( any const & inArg1, any const & inArg2 )
    {
        T1 obj1 = any_cast< T1 >( inArg1 );
        T2 obj2 = any_cast< T2 >( inArg2 );
        std::cout << "-------\n";
        std::cout << "Arg1 : " << obj1 << std::endl;
        std::cout << "Arg2 : " << obj2 << std::endl;
        return obj2;
    }

    bool IsOfType( std::type_info const * inT1Ptr,
            std::type_info const * inT2Ptr )
    {
        if( *inT1Ptr == typeid( T1 ) and *inT2Ptr == typeid( T2 ) )
            return true;
        else
            return false;
    }
};

//------------------------------------------------------------

class CFactory
{
public:
    static CFactory & GetInstance()
    { static CFactory sObj; return sObj; }

    template< typename T1, typename T2 >
    shared_ptr< CObjBase > Get();

private:

    typedef std::vector< shared_ptr< CObjBase > > CRegister;

    CRegister mRegister;

    CFactory() {}
    CFactory( CFactory const & );
    ~CFactory() {}
    CFactory & operator=( CFactory const & );
};

CFactory & Factory() { return CFactory::GetInstance(); }

//------------------------------------------------------------

template< typename T1, typename T2 >
shared_ptr< CObjBase > CFactory::Get()
{

    CRegister::iterator iter = std::find_if(
        mRegister.begin(),
        mRegister.end(),
        bind( &CObjBase::IsOfType, _1, &typeid( T1 ), &typeid( T2 ) )
    );

    if( iter == mRegister.end() )
    {
        std::cout << "CREATING\n";
        shared_ptr< CObjBase > ptr( new CObjConcrete< T1, T2 > );
        mRegister.push_back( ptr );
        return ptr;
    }
    else
    {
        std::cout << "REUSING\n";
        return *iter;
    }
}

//------------------------------------------------------------

int main()
{
    Factory().Get< int, double >()->DoSome( 13.56 );
    Factory().Get< std::string, int >()->DoOther(
        std::string( "ola" ),
        100
    );
    Factory().Get< int, double >()->DoSome( 67.67 );
    Factory().Get< std::string, int >()->DoOther(
        std::string( "TEST" ),
        900
    );
    std::cin.get();
}

//end code


Thanks Francesco,

This is exactly what I was looking for. I have a quick question about
your IsOfType function:

bool IsOfType( std::type_info const * inT1Ptr,
        std::type_info const * inT2Ptr )
{
    if( *inT1Ptr == typeid( T1 ) and *inT2Ptr == typeid( T2 ) )
        return true;
    else
        return false;
}

You are using 'and' is that in the boost namespace? (I couldnt find it),
or did you simply mean the unary boolean 'and' operator (&&) ?


[this might be a double post... sorry in advance]

Anyway, no 'and' is not boost stuff, it's standard c++: it's just an
alternative token to &&.
Check out the standard @ "2.5 Alternative tokens".
I believe it's a binary op though... ;-)
Glad I helped, bye,
Francesco

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