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.
Yes, this operator implementation doesn't make much sense. I'd recommend:
bool operator<(const myStruct& rhs) const {
return rhs.a < this->a || (rhs.a == this->a && rhs.b < this->b);
}
bool operator<(const myStruct& rhs) const {
if (a < rhs.a) return true;
if (a == rhs.a) return b < rhs.b;
return false;
}
If you are looking for a generalization to many data members, there is a great example using C++11 std::tie:
struct S {
int n;
std::string s;
float d;
bool operator<(const S& rhs) const {
return std::tie(n, s, d) < std::tie(rhs.n, rhs.s, rhs.d);
}
};
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