How can I use a C++ unordered_set for a custom class?

Question

How can I store objects of a class in an unordered_set? My program needs to frequently check if an object exists in this unordered_set and if it does, then do some update on that object.

I have looked up online on how to use unordered_set, but sadly most tutorials are about using it on int or string types. But how can I use it on a class? How can I define a hash function to make the node_id in the following example the key of the unordered_set?

#include <iostream> #include <unordered_set>  using namespace std;  // How can I define a hash function that makes 'node' use 'node_id' as key?     struct node {     string node_id;     double value;     node(string id, double val) : node_id(id), value(val) {} };  int main() {     unordered_set<node> set;     set.insert(node("1001", 100));     if(set.find("1001") != set.end()) cout << "1001 found" << endl; } 

Answer 1

Since this is the top Google result on Stack Overflow for C++ unordered_set of objects I'll post a simple yet completely illustrative and copy/paste runnable example:

// UnorderedSetOfObjects.cpp  #include <iostream> #include <vector> #include <unordered_set>  struct Point {   int x;   int y;    Point() { }   Point(int x, int y)   {     this->x = x;     this->y = y;   }      bool operator==(const Point& otherPoint) const   {     if (this->x == otherPoint.x && this->y == otherPoint.y) return true;     else return false;   }    struct HashFunction   {     size_t operator()(const Point& point) const     {       size_t xHash = std::hash<int>()(point.x);       size_t yHash = std::hash<int>()(point.y) << 1;       return xHash ^ yHash;     }   }; };  int main(void) {   std::unordered_set<Point, Point::HashFunction> points;    points.insert(Point(1, 1));   points.insert(Point(2, 2));   points.insert(Point(1, 1));   // notice this is a duplicate with the 1st point so it won't change the set    std::cout << "points: " << "\n";   for (auto& point : points)   {     std::cout << "(" << point.x << ", " << point.y << ")" << "\n";   }    return 0; } 

Answer 2

You could try using the following hash function object (it's pretty basic so you may want to improve it to avoid too many collisions).

struct node_hash {     std::size_t operator()(const node& _node) const {         return std::hash<std::string>()(_node.node_id);     } } // ... std::unordered_set<node, node_hash> node_set; 

However, as one of the comments points out, you may be better off using a std::unordered_map<std::string, double> here.