algorithm - C++: Using boost lambda to get maximum values in a std::tr1::unordered_map -

i have std::tr1::unordered_map<int, a> map; class member variable (int x).

i find key in map such map[i].x maximum.

i know can write functor go std::max_element. how do instead, using boost lambda (i trying learn it) ? not have c++0x.

as added question, if had class defined below:

class { int x; int y; };

and wanted find maximum w.r.t x (if maximum 0, find maximum wrt y). again, 1 solution iterate on tuples of map (keeping both maxima in memory). there way modify max_element ?

boost.lambda uses boost.bind access member variables. comes bit wordy:

typedef std::tr1::unordered_map<int, a> map_t; map_t m;  max_a = std::max_element(m.begin(), m.end(),                 bind(&a::x, bind(&map_t::value_type::second, _1))               < bind(&a::x, bind(&map_t::value_type::second, _2))          )->second; 

test: https://ideone.com/v6szl

you may save half headache boost.range

a max_a = *boost::max_element(m | map_values, bind(&a::x, _1) < bind(&a::x, _2)); 

but in practice, think, functor best.