Re: Heterogeneous collection: return type overload

From:
DeMarcus <use_my_alias_here@hotmail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 20 Feb 2010 14:45:07 +0100
Message-ID:
<4b7fe764$0$279$14726298@news.sunsite.dk>
jrbcast wrote:

On Feb 18, 9:53 pm, "Leigh Johnston" <le...@i42.co.uk> wrote:

"jrbcast" <jrbc...@gmail.com> wrote in message

news:866a2791-2934-41a3-ba19-6721111f887a@u9g2000yqb.googlegroups.com...

On Feb 18, 9:04 pm, James Kanze <james.ka...@gmail.com> wrote:

On Feb 18, 6:50 am, jrbcast <jrbc...@gmail.com> wrote:

On Feb 17, 7:59 pm, "Leigh Johnston" <le...@i42.co.uk> wrote:

"jrbcast" <jrbc...@gmail.com> wrote in message
news:ae30bbcc-2fea-413d-9696-ad036be5d29d@f15g2000yqe.googlegroups.com...

Imagine I have something like this:
class Collection
{
std::map<char *, void *> elements;

Using a char* as a key in a map is probably not a very good
idea.

Yes, I realized about that after posting ;-), I am now using
std::string

Collection();

Try "std::map<std::string, boost::any> elements;" instead.

Thanks for your responses. Now it is clear that C++ does not
support function overloading on return types :-(.

Not directly, but it's fairly simple to get the same effect by
means of a proxy:
    class Collection
    {
    public:
        class Proxy
        {
            Collection const* owner;
            std::string key;
        public:
            Proxy(Collection const& owner, std::string const& key)
                : owner(&owner)
                , key(key)
            {
            }
            template< typename T > operator T() const
            {
                return owner->... // whatever it takes to get
                                   // the correctly typed value
            }
        }
        Proxy get(std::string const& key) const
        {
            return Proxy(*this, key);
        }
        // ...
    };

I am afraid I am not able to follow the code (too hard for me and my
usual needs). Can you point me to some book/web... where to learn such
special things?
Cheers

Which bit(s) don't you understand?

/Leigh


Thank you very much, I did not realize that "proxy" was the key word
to google it :-S. Sorry, too late here after work (Spain = 21:54).

I can not imagine how this proxy hides types management in a
heterogeneous structure like that I am proposing here std::map<
std::string, void *> so the caller does not need to explicitly pass
the type it is expecting, just receive it (the called object will care
about that)...

I have written something like this but I would like (if possible) to
know if there is a more "advanced" or "correct" way to do that:

// PLEASE NOTE CODE HAS NOT BEEN COMPILED YET AND MAY BE ERRONEOUS

#define angle_t double

#define rot(object) *((angle_t*)(object.getValue(std::string("rot"))))
#define tilt(object) *((angle_t*)
(object.getValue(std::string("tilt"))))
#define psi(object) *((angle_t*)(object.getValue(std::string("psi"))))

class objectValues
{
    std::map<std::string, void *> values;
    objectValues();
    ~objectValues();

    int addValue( std::string name, int value )
    {
        void * newValue = (void *)(new int(value));
        return insertVoidPtr( name, newValue );
    }

    int addValue( std::string name, double value )
    {
        void * newValue = (void *)(new double(value));
        return insertVoidPtr( name, newValue );
    }

    int insertVoidPtr( std::string name, void * value )
    {
        // Return value for "insert" call
        pair<map<std::string, void *>::iterator,bool> ret;

   ret = values.insert( pair<std::string, void*>(name,value) );

        if (ret.second==false)
        {
            return -1;
        }
        else
        {
            return 0;
        }
    }

    void * getValue( std::string name )
    {
        map <std::string, *void>::const_iterator element;

    element = values.find( name );

    if ( element == values.end( ) )
        {
            return NULL;
        }
        else
        {
            return element->second;
        }
    }

    bool valueExists( std::string name )
    {
        map <std::string, *void>::const_iterator element;

    element = values.find( name );

    if ( element == values.end( ) )
        {
            return false;
        }
        else
        {
            return true;
        }
    }

    void deleteValue( std::string name )
    {
        map <std::string, *void>::const_iterator element;

    element = values.find( name );

    if ( element != values.end( ) )
        {
            values.erase( element );
        }
    }
}


You may be in the same situation as I have at a couple of places, where
I need to store values of any type in a container to be able to process
them later.

Following is a simplification of my own solution, but please note that I
have not found this solution anywhere on the net or in any book, so I
don't know if this is the right way to go. However, it seems to work
pretty good so far. The solution is built on the Visitor pattern.
http://en.wikipedia.org/wiki/Visitor_pattern

In this solution you first have to identify the receiver of your object
values. I.e. it may not be the perfect solution if you want to create a
generic map that can deliver your objects to whatever. In my case I
wanted to deliver a container of any type to std::cout. Basically I
wanted to create a container with contents like this
{ "Hello ", myOwnObject, " number ", 7, std::endl } and I couldn't
calculate it during the call, hence, I first needed to store the values
in a container until they should be used.

Ok, back to step one. I identified the receiver as std::cout. Then I
created an objectValue-to-receiver interface like this.

class ObjectValueInterface
{
public:
    virtual ~ObjectValueInterface() {}
    virtual void applyTo( std::ostream& ) = 0;
    // applyTo() would represent accept() in the Visitor pattern.
};

// Then create a template for any object value like this.
// Here you also need to decide whether you want a copy of your object
// or a (smart) pointer to your object. Since you copy in your own
// example I do the copy variant here.

template<typename T>
class ObjectValue : public ObjectValueInterface
{
public:
    ObjectValue( T objVal ) : objVal_(objVal) {}
    virtual void applyTo( std::ostream& os )
    {
       os << objVal_;
       // operator<<() would represent visit() in the Visitor pattern.
    }
private:
    T objVal_;
};

// Now you create your object value container.
#include <map>

class ObjectValueContainer
{
public:
    template<typename T>
    int addValue( const std::string& key, T val )
    {
       if( objVals_.find( key ) != objVals_.end() )
          return -1;

       objVals_[key] = new ObjectValue<T>( val );

       return 0;
    }

    ObjectValueInterface* getValue( const std::string& key )
    {
       if( objVals_.find( key ) == objVals_.end() )
          return NULL;

       return objVals_[key];
    }
private:
    std::map<std::string, ObjectValueInterface*> objVals_;

// Note! This is a simplified example. You should consider using
// boost::shared_ptr instead of a plain pointer here.
// boost::shared_ptr will be std::shared_ptr with the next
// standard. Until then, see
// www.boost.org/doc/libs/1_42_0/libs/smart_ptr/shared_ptr.htm

};

// Now use it.
// Note! In this example we skip error handling.

int main()
{
    ObjectValueContainer objVals;

    // Add some values.
    objVals.addValue( "Name", "Daniel" );
    objVals.addValue( "House Number", 3 );
    objVals.addValue( "Double", 47.11 );
    objVals.addValue( "Delimiter", " " );

    // Print the values.
    objVals.getValue( "Name" )->applyTo( std::cout );
    objVals.getValue( "Delimiter" )->applyTo( std::cout );
    objVals.getValue( "House Number" )->applyTo( std::cout );
    objVals.getValue( "Delimiter" )->applyTo( std::cout );
    objVals.getValue( "Double" )->applyTo( std::cout );

    return 0;
}

Hope this helps!
If anyone else have comments on this kind of solution I would be happy
to hear it so I can find flaws and improve it.

/Daniel

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