using C++ std::sort with lambda comparator [duplicate]

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• Operator overloading 7 answers

I'm tryin to sort an array of objects using std::sort -

sort(convexHull.getSet(), convexHull.getSet()+convexHull.size(),
     [](const Point & a, const Point & b) -> bool
     {             if (a.getX() < b.getX())
         return true;
     else if (a.getX() == b.getX())
         return a.getY() < b.getY();
     else
         return false;; }
);

as explained here.

But i'm getting a really long error from my compiler (Clion), something about my assignment operator -

Point& Point::operator= (const Point &rval)
{
    _x = rval.getX();
    _y = rval.getY();
    setArg();
    return *this;
}

(where setArg simply computes a point angle from the positive x-axis).

i've tested this code and it works fine.

it's very long so i'll post just part of it -

In file included from /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:61:0,
                 from /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/algorithm:62,
                 from /cygdrive/c/Users/o/Desktop/CPP/ex1/cppex1/ex1/ConvexHull.cpp:7:
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_heap.h: In instantiation of 'void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = const Point*; _Distance = long int; _Tp = Point; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]':
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_heap.h:333:22:   required from 'void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = const Point*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:1669:23:   required from 'void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = const Point*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:1928:25:   required from 'void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = const Point*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:1943:27:   required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = const Point*; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:1963:25:   required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = const Point*; _Compare = __gnu_cxx::__ops::_Iter_comp_iter<main()::<lambda(const Point&, const Point&)> >]'
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_algo.h:4729:18:   required from 'void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = const Point*; _Compare = main()::<lambda(const Point&, const Point&)>]'
/cygdrive/c/Users/o/Desktop/CPP/ex1/cppex1/ex1/ConvexHull.cpp:74:5:   required from here
/usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/bits/stl_heap.h:218:29: error: passing 'const Point' as 'this' argument discards qualifiers [-fpermissive]
    *(__first + __holeIndex) = _GLIBCXX_MOVE(*(__first + __secondChild));
                             ^
In file included from /cygdrive/c/Users/o/Desktop/CPP/ex1/cppex1/ex1/PointSet.h:9:0,
                 from /cygdrive/c/Users/o/Desktop/CPP/ex1/cppex1/ex1/ConvexHull.cpp:4:
/cygdrive/c/Users/o/Desktop/CPP/ex1/cppex1/ex1/Point.h:45:12: note:   in call to 'Point& Point::operator=(Point&)'
     Point& operator= (Point &rval);