Declaring function objects for comparison?

Question

I have seen other people questions but found none that applied to what I'm trying to achieve here.

I'm trying to sort Entities via my EntityManager class using std::sort and a std::vector<Entity *>

/*Entity.h*/
class Entity
{
public:
 float x,y;
};

struct compareByX{
 bool operator()(const GameEntity &a, const GameEntity &b)
 {
  return (a.x < b.x);
 }
};   
   
/*Class EntityManager that uses  Entitiy*/

typedef std::vector<Entity *> ENTITY_VECTOR; //Entity reference vector
   
class EntityManager: public Entity
{
private:
 ENTITY_VECTOR managedEntities;

public:
 void sortEntitiesX();
};

void EntityManager::sortEntitiesX()
{
 
 /*perform sorting of the entitiesList by their X value*/
 compareByX comparer;
 
 std::sort(entityList.begin(), entityList.end(), comparer);
}

I'm getting a dozen of errors like

: error: no match for call to '(compareByX) (GameEntity* const&, GameEntity* const&)'
: note: candidates are: bool compareByX::operator()(const GameEntity&, const GameEntity&)

I'm not sure but ENTITY_VECTOR is std::vector<Entity *> , and I don't know if that could be the problem when using the compareByX function object ?

I'm pretty new to C++, so any kind of help is welcome.

Answer 1

And a third one comes in... After you edited you question, still one open topic: your comparator takes a const & to the GameEntity class. It should, in order to work with the values of the vector<GameEntity*>, take const GameEntity* arguments instead.