Question

I’m making a program that generates a maze and then uses bredth first search to find a way in the maze. My function that checks if an element is present in a container-class now uses the vector like this (where coordinatePath is a typedef for vector) :

bool Labyrinth::inVisited(const Coordinate &c, const coordinatePath &visited ) const
{
    for each (Coordinate coord in visited)
    {
        if(coord == c)
            return true;
    }
    return false;
}

Since this method has to traverse the full container if an element is not present it’s very ineffective for large searches. I tried to implement the same function that uses a set instead of a vector and wrote it like this:

bool Labyrinth::inVisited(const Coordinate &c, const set<Coordinate> &visited ) const
{
       return (visited.find(c) != visited.end());
}

when i try to recomplie i get a lot of errors where the topmost is

Error 22 error C2676: binary ‘<‘ : ‘const Coordinate’ does not define this operator or a conversion to a type acceptable to the predefined operator c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstddef 193

I don’t really understand these particular debug-messages and wonder if there is a way to implement this faster search!

Answer 1

To use elements in a set the value_type has to define operator< or you need to provide a comparison functor to the container. Apparently, your Coordinate type doesn’t do that or the operator< you provide takes incompatible arguments. It should look roughly like this:

struct Coordinate {
  bool operator<(const Coordinate& other) const { return false; }
};

// or by providing a functor
struct CmpCoord {
  bool operator()(const Coordinate& x, const Coordinate& y);
}; 
typedef std::set<Coordinate, CmpCoord> coord_set;