Accessing a nested pair

Question

To take apart a pair, the following can be done

boost::bind(&std::pair::second, _1); // returns the value of a pair

What about using combinations of different containers, how can a nested pair be accessed?

For example when I wanted to partition a vector into items contained in a supplemental map and items that where not contained in the supplemental map I used the following:

typedef int DWORD; typedef std::pair<std::string, bool> user_info; 
typedef std::map<DWORD, user_info> USER_MAP; 
typedef std::vector<DWORD> VEC_STAFF; 
VEC_STAFF::iterator it = std::partition(
   Staff.begin(), Staff.end(), 
  (bind(&USER_MAP::find, m_Users, _1) != m_Users.end()));

Now I have a second problem - during the running of the application the status bool of user_info can change, and later on I want to re-partition the vector with items that have a status bool of true rather than just being contained in the supplemental map.

However I seem to have a problem accessing the second item of a nested pair.

I tried the following but I cannot seem to access the nested pair!

VEC_STAFF::const_iterator itCurEnd = partition(Staff.begin(), Staff.end(),
     bind(&USER_MAP::value_type::second::second, 
     bind(&USER_MAP::find, &m_Users, _1)) == true);

Answer 1

The syntax you used obviously doesn't work. The first "::second" already signifies a non-static member and not a type. If you have a pair inside a pair you probably have to use two bind calls:

typedef std::pair< int, bool > foo_t;
typedef std::pair< int, foo_t > bar_t;
.....

bind( &foo_t::second, bind(&bar_t::second,
    bind( &USER_MAP::find, _1 )
) )

(I didn't test this. Perhaps this is what you want)

But three levels if bind is quite challenging, in my opinion.

Edit: How about this?

template<typename Iter, typename Return>
struct deref_2nd_2nd : std::unary_function<Iter, Return> {
    Return operator()(Iter const& it) const {
        return (*it).second.second;
    }
};

.....

bind(
    deref_2nd_2nd<USER_MAP::iterator,bool>(),
    bind( &USER_MAP::find, _1 )
)

Answer 2

Not sure I follow what's going on there, but usually when I start running into problems with bind I give up and implement a functor. This might simplify things in your case.

To me, the following is a lot easier to read than all that messing around with multiple levels of binds

template <class Arg>
struct Whatever : public std::unary_function<Arg, bool>
{
    bool operator()(const Arg &arg1) const {
        //logic here
    } 
};

Whatever<???> functor;
std::partition(Staff.begin(), Staff.end(), functor);