map choose `std::greater` or `std::less` at runtime

Question

This line:

std::map<long int, long int, std::greater<long int>> current_book;

I would like to replace it by the logical equivalent of:

int Side = ...
if (Side == 1){
    std::map<long int, long int, std::greater<long int>> current_book;
} else {
    std::map<long int, long int, std::less<long int>> current_book;
}

Answer 1

You can use std::function for that:

using mymap = std::map<long,long,std::function<bool(long,long)>>;
auto m = Side ? mymap( std::less<long>() ) : mymap( std::greater<long>() );

live example

Answer 2

You have to implement a custom comparator class, something along the lines of:

class map_comparator {

   bool less_than;

public:

   map_comparator(bool less_than) : less_than{less_than} {}

   bool operator()(long a, long b) const
   {
       if (less_than)
             return a < b;
       return a > b;
   }
};

And then use it to construct your map, using the constructor that takes an instance of the comparator class as a parameter, and then pass in the appropriately constructed comparator class instance:

std::map<long int, long int, map_comparator>
     current_book{ map_comparator{ Side != 1}};

Answer 3

You can use a straight function pointer. This approach won't suffer from overhead of if (less_than) branching on each comparison or from overhead of std::function:

#include <map>

using t_Key = long int;
using t_Value = long int;
using t_Comparator = bool ( * )(t_Key const left, t_Key const right);
using t_Map = ::std::map<t_Key, t_Value, t_Comparator>;

bool Is_Less(t_Key const left, t_Key const right)
{
    return left < right;
}

bool Is_Greater(t_Key const left, t_Key const right)
{
    return left > right;
}

int main()
{
    int Side{};
    t_Map current_book{(Side == 1) ? &Is_Less : &Is_Greater};
    return 0;
}