C++: how to use std::less<int> with boost::bind and boost::lambda?

Question

I am trying to lean boost::bind, boost::lambda libraries and how they can be used with STL algorithms. Suppose I have vector of int-string pairs which is sorted by int key. Then a place to insert a new pair while keeping the vector sorted can be found as follows:

std::vector<std::pair<int, string> > entries;
...
int k = ...;

// Let's ignore std::lower_bound return value for now
std::lower_bound (entries.begin(), entries.end(), k, 
                  boost::bind (&std::pair<int, string>::first, _1) < k)

Now I would like to replace operator< with a function object (of type std::less<int> in this example):

std::less<int> comparator;

How do I change the code above so it works? I cannot just do

std::lower_bound (entries.begin(), entries.end(), k, 
                  comparator (boost::bind (&std::pair<int, string>::first, _1), k))

because std::less<int>::operator() does not accept whatever is the return type of boost::bind. What am I missing here? TIA

Answer 1

All you're missing is another bind (and the template parameters on pair):

std::lower_bound(entries.begin(), entries.end(), k, 
                 boost::bind(comparator,
                             boost::bind(&std::pair<int, string>::first, _1),
                             k))

You don't have to do that on the less-than operator in your original code because Boost.Bind provides overloads for that operator that know how to handle the return type of boost::bind.