how to compare structs

Question

I am having difficulties to set up the comparison correctly. Here is an example of my problem, where my code wrongly assumes {1,2}={2,1}: http://ideone.com/i7huL

#include <iostream>
#include <map>
using namespace std;

struct myStruct {
  int a;
  int b;
  bool operator<(const myStruct& rhs) const {
           return rhs.a < this->a && rhs.b < this->b;
  }
};


int main() {
       std::map  <myStruct, int> mymap ;
       myStruct m1={1,2};
       myStruct m2={2,1};
       mymap.insert(make_pair(m1,3));
       std::map<myStruct, int>::iterator it1 = mymap.find(m1);
       std::map<myStruct, int>::iterator it2 = mymap.find(m2);
       cout << it1->second << it2->second;
       // here it1->second=it2->second=3, although I would have expected it2 to be equal to map.end().
}

I could use || instead of &&, but I'm not sure this is the correct way either. I just want to have operator< implemented in such a way that I am able to find objects in my map, without making any errors, as is the case in the code I linked to.

Thanks.

Answer 1

I prefer to write this by comparing elements for equality until two are found that are different:

bool operator<(const myStruct& rhs) const {
    if (a != rhs.a)
        return a < rhs.a;
    if (b != rhs.b)
        return b < rhs.b;
    return false; // this and rhs are equal.
}

I find this clearer and more extensible than writing a single expression with a mix of || and && (as per @HansPassant), and more compact than @jahhaj's approach of having each passing test lead to a return true; or return false;. Performance is about the same, unless you know something about the distribution of values. There is an argument for avoiding operator==() and just using operator<(), but that only applies if you are trying to write maximally generic template code.