Iterator adapters and maps

From: Alisdair Meredith (yg-boost-users_at_[hidden])
Date: 2002-10-02 14:43:23

I have a std::map<std::string, MyObject> and want to iterate over it
calling several member functions of MyObject.  This seems like a good
case to learn about transform/projection iterator, but I am having a
hard time getting a compiling example.

My most recent attempt looks like this:

class MyObject
{
public:
  void CommitChanges();
  void RollbackChanges();
  // More implementation details would follow
};

typedef std::map<std::string, MyObject> MyMap;
typedef MyMap::iterator    MyIterator;
typedef MyMap::value_type  mapped_type;

typedef boost::projection_iterator_generator< Select2ndRef<mapped_type>,
MyIterator >::type  projection;

MyMap theMap;

projection begin( theMap.begin() );
projection end( theMap.begin() );
for_each(   begin, end, mem_fun_ref( MyObject::CommitChanges ) );

I also have the following two templates, although I suspect their
equivalent must be lurking in boost somewhere:

template<class pair_type>
struct  Select1stRef   
{
    typedef typename pair_type::first_type &    result_type;
    result_type operator()( pair_type &source ) const
    {
        return  source.first;
    }
};

template<class pair_type>
struct  Select2ndRef   
{
    typedef typename pair_type::second_type &   result_type;
    result_type operator()( pair_type &source ) const
    {
        return  source.second;
    }
};

I am compiling using BCB5 compiler in the IDE, should be equivalent to
the free 5.5.1 command line compiler.

While I could simply write a forwarding functor in a fraction of the
time it will take me to solve this, there is definitely a ne tool I
would like to add to my toolbox here once I understand what is
happenning.  Unfortunately I am getting somewhat lost in
reference-to-reference issues, and the occasional compiler crash does
not help.

Any and all hints appreciated 

-- 
AlisdairM