c++ - Sort objects in descending order when the < comparator is defined? -

i have class a , < comparator. how can use them sort array of a in descending order?

class { ... };  class lessa {    bool operator()(const a& a1, const a& a2) const {    ...    } }  vector<a> v; sort(v.begin(), v.end(), ???); 

i suppose should replace ??? based on lessa, can't figure out should go in there. thought of using lambda function, looking shorter.

if want sort according relation defined lessa comparator, pass instance of lessa third argument (and, since using c++11, prefer global std::begin() , std::end() functions):

std::sort(std::begin(a), std::end(a), lessa()); //                                    ^^^^^^^ 

now if lessa() expresses < relation , want sort according opposite criterion, do:

std::sort(std::begin(a), std::end(a),      [] (a const& a1, const& a2)) {     return lessa()(a2, a1); } 

another thing let custom comparator accept argument determines how should perform comparison:

class compa {     bool lessthan; public:     compa(bool lessthan) : _lessthan(lessthan) { }     bool operator()(const a& a1, const a& a2) const {         if (_lessthan)         {             // return true iff a1 < a2;         }         else         {             // return true iff a1 > a2;         }     } }; 

you use way sort in ascending order:

std::sort(std::begin(a), std::end(a), compa(true)); 

and way sort in descending order:

std::sort(std::begin(a), std::end(a), compa(false)); 

another possibility, given original lessa comparator, use std::bind swap order of arguments custom comparator:

lessa comp; using namespace std::placeholders; std::sort(std::begin(v), std::end(v),      std::bind(&lessa::operator(), comp, _2, _1));