STL set custom sort

Question

I was trying to resort a set when i realised that it was impossible to resort a set and i had to create a new set and have a custom sort function to resort it . I researched onl

Answer 1

Your two sets have different types (the compare function is part of the type definition).

Since std::set does not provide a constructor or assignment operator for different compare functions, you have to either initialize the new set with an iterator pair to your first set or std::copy over the elements.

That is:

set<Point2D,p2d_sortby_y> p2d_set2 { std::begin(p2d_set), std::end(p2d_set) };

or

set<Point2D,p2d_sortby_y> p2d_set2;
std::copy(std::begin(p2d_set), std::end(p2d_set), std::inserter(p2d_set2));

Answer 2

In set<Point2D, p2d_sortby_y> the second parameter p2d_sortby_y does not name a type.

struct point_odered_by_y
{
    bool operator()(const Point2D& ptd1, const Point2D& ptd2) const
    {
        /* .. */
    }
};



typedef std::set<Point2D, point_odered_by_y> pointset;
pointset s(begin(p2d_set), end(p2d_set));

Answer 3

This would be an easier way to do it:

#include <tuple>

struct SortByYX
{
  bool operator ()(const Point2D& lhs, const Point2D& rhs) const
  {
    return std::tie(lhs.y, lhs.x) < std::tie(rhs.y, rhs.x);
  }
};

Then

set<Point2D, SortByYX> p2d_set2(p2d_set.begin(), p2d_set.end());

Edit: std::tie requires C++11 support, but if you don't have it you can use std::tr1::tie from <tr1/tuple>, or boost::tie if you don't have TR1.

Answer 4

It's usually better to define a functor class to specify the ordering, rather than a function; that way, you can specify it as a template parameter:

struct p2d_sortby_y {
    bool operator()(Point2D x, Point2D y) {return whatever;}
};

This has to take its arguments by value or const reference (not non-const reference, as yours does) to be used on the constant set members.

You can't directly initialise the new set from a different type of set, but you can initialise it from its range:

set<Point2D,p2d_sortby_y> p2d_set2(p2d_set.begin(), p2d_set.end());