populating vector into map - c++

Question

I´m trying to populate a Point map with a vector of Points. I´m trying to make a board game where each position on the board has a point (x,y) and vector of legal moves (Point objects).
I can´t seem to be able to have a map KEY as a Point.
struct Point
{
Point() {}
Point(int ix, int iy ) :x(ix), y(iy) {}
int x;
int y;
};
Point p_source (2,2);
Point p_next1 (1,2);
Point p_next2 (1,3);
Point p_next3 (1,4);
map <Point, vector<Point> > m_point;
dict[p_source].push_back(p_next1);
dict[p_source].push_back(p_next2);
dict[p_source].push_back(p_next3);
This is the errors that I get
In member function 'bool std::less<_Tp>::operator()(const _Tp&, const
_Tp&) const [with _Tp = Point]':|
instantiated from '_Tp& std::map<_Key, _Tp, _Compare,
_Alloc>::operator[](const _Key&) [with _Key = Point, _Tp = std::vector,
std::allocator >, std::allocator, std::allocator > > >, _Compare =
std::less, _Alloc = std::allocator,
std::allocator >, std::allocator, |
instantiated from here|
c:\program files ( no match for 'operator<' in '__x < __y'| ||===
Build finished: 1 errors, 0 warnings ===|

Answer 1

Checking my favourite online reference it reads:
template<
class Key,
class T,
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T> >
> class map;
Map is an associative container that contains a sorted list of unique
key-value pairs. That list is sorted using the comparison function
Compare applied to the keys. Search, removal, and insertion operations
have logarithmic complexity. Maps are usually implemented as red-black
trees.
Since you don't provide an explicit Compare it sorts using the default std::less<Key>. Seems like we're on the right track because the errors are in that class:
In member function 'bool std::less<_Tp>::operator()(const _Tp&, const
_Tp&) const [with _Tp = Point]':|
Let's check that:
template< class T >
struct less;
Function object for performing comparisons. Uses operator< on type T.
This matches what the error messages tell us:
no match for 'operator<' in '__x < __y'
Hmm, but there's no operator< for type Point...

Answer 2

Your error is entirely unrelated to std::vector<> – std::map<> requires that its key either be comparable with operator<, or that you supply a custom comparitor. The easiest solution is to add the following after Point's definition:
bool operator <(Point const& lhs, Point const& rhs)
{
return lhs.y < rhs.y || lhs.y == rhs.y && lhs.x < rhs.x;
}