无法使用struct作为键在C ++映射中查找插入的值

Question

I'm trying to use a map to store map information using coordinates x,y as key. I cant iterate properly trough this map using the auto iterator or map.find(). It just wont return me the correct map value. I'm using C++14, here is my chunk of code:

#include <iostream>
#include <string>
#include <map>
#include <iterator>
#include <vector>

using namespace std;

struct mapPoint {
  int X;
  int Y;

  bool operator < (const mapPoint &coord) const {
    if (X == coord.X && Y == coord.Y) {
      return true;
    } else {
      return false;
    }
  }
};

struct explorerNotes {
  bool foundCoin;
  int timesVisitedThisBlock;
  vector<bool> unexploredEntrances; // 0 0 0 0 -> N S E W
  explorerNotes():timesVisitedThisBlock(0),unexploredEntrances(4,false){}
};

int main() {
  map<mapPoint, explorerNotes> explorerNotebook;

  explorerNotes testNote1, testNote2, testNote3;
  mapPoint testCoord1, testCoord2, testCoord3;

  testNote1.foundCoin = true;
  testNote1.timesVisitedThisBlock = 42;
  testNote1.unexploredEntrances = {true, true, false, false};
  testCoord1.X = 25;
  testCoord1.Y = 3;

  testNote2.foundCoin = false;
  testNote2.timesVisitedThisBlock = 314;
  testNote2.unexploredEntrances = {false, true, false, false};
  testCoord2.X = 11;
  testCoord2.Y = 2;

  testNote3.foundCoin = true;
  testNote3.timesVisitedThisBlock = 420;
  testNote3.unexploredEntrances = {false, true, false, false};
  testCoord3.X = 1;
  testCoord3.Y = 1;

  explorerNotebook.insert(pair<mapPoint, explorerNotes>(testCoord1, testNote1));
  explorerNotebook.insert(pair<mapPoint, explorerNotes>(testCoord2, testNote2));
  explorerNotebook.insert(pair<mapPoint, explorerNotes>(testCoord3, testNote3));

  map<mapPoint, explorerNotes>::iterator p;
  p = explorerNotebook.find(testCoord1);
  cout << " testing 1:"
       << "\nfoundCoin: " << p->second.foundCoin
       << "\ntimesVisitedThisBlock: " << p->second.timesVisitedThisBlock
       << "\nunexploredEntrances: "<< "(" << p->second.unexploredEntrances[0] << "," << p->second.unexploredEntrances[1] << "," << p->second.unexploredEntrances[2] << "," <<p->second.unexploredEntrances[3] << ")" << endl;

  map<mapPoint, explorerNotes>::iterator q;
  q = explorerNotebook.find(testCoord2);
  cout << " testing 2:"
       << "\nfoundCoin: " << q->second.foundCoin
       << "\ntimesVisitedThisBlock: " << q->second.timesVisitedThisBlock
       << "\nunexploredEntrances: "<< "(" << q->second.unexploredEntrances[0] << "," << q->second.unexploredEntrances[1] << "," << q->second.unexploredEntrances[2] << "," <<q->second.unexploredEntrances[3] << ")" << endl;

  map<mapPoint, explorerNotes>::iterator r;
  r = explorerNotebook.find(testCoord3);
  cout << " testing 3:"
       << "\nfoundCoin: " << r->second.foundCoin
       << "\ntimesVisitedThisBlock: " << r->second.timesVisitedThisBlock
       << "\nunexploredEntrances: "<< "(" << r->second.unexploredEntrances[0] << "," << r->second.unexploredEntrances[1] << "," << r->second.unexploredEntrances[2] << "," <<r->second.unexploredEntrances[3] << ")" << endl;;

  return 0;
}

When I compile this code it gave me the output:

 testing 1:
foundCoin: 1
timesVisitedThisBlock: 42
unexploredEntrances: (1,1,0,0)
 testing 2:
foundCoin: 1
timesVisitedThisBlock: 42
unexploredEntrances: (1,1,0,0)
 testing 3:
foundCoin: 1
timesVisitedThisBlock: 42
unexploredEntrances: (1,1,0,0)

It just repeats the first added value...I'm stuck all day. Any ideas? Thanks in advance.

Answer 1

Your operator < is completely messed up.

You are saying that A is less than B if its X coordinate is equal and its Y coordinate is equal. So (1,1) < (2,2) is false, but (1,1) < (1,1) is true. No wonder the map can't manage to find the right entry.

In particular:

• since (1,1) < (2,2) is false, and (1,1) > (2,2) is false (> being < with the arguments reversed), that means (1,1) and (2,2) must be equal!
• since (1,1) < (1,1) is true, (1,1) > (1,1) can't be true, because one thing can't be less than and greater than another thing. Yet it is.

You need to figure out an actual order for the points, and implement that order in operator < . For example, you could say that (a,b) < (c,d) if a<c, or if a==c and b<d. Then you get that (1,1) < (1,2) < (1,3) < (2,0) < (2,1) < ...

bool operator < (const mapPoint &coord) const {
    if (X < coord.X || (X == coord.X && Y < coord.Y)) {
        return true;
    } else {
        return false;
    }
}

Answer 2

Your < does not qualify as a strict weak ordering. So your code does UB once put in the map.

The easy way to write a conforming < is:

friend auto mytie(const mapPoint& self){
  return std::tie(self.X, self.Y);
}
friend bool operator<(const mapPoint& lhs, const mapPoint& rhs){
  return mytie(lhs)<mytie(rhs);
}

Where we delegate < to tuple 's implementation without copies and with maximal DRY (don't repeat yourself).

I find doing < manually is easy, but error prone over the decades.