Custom comparator for std::map not working

Question

I experienced the following phenomenon when writing some C++ code:

I have a map looking like this:

std::map<test_struct_t*, unsigned int, cmp_by_value> testmap;

This map lies globally in my program and the struct is defined as:

struct test_struct_t {
  int x; int y; int val;

  bool operator<(const test_struct_t &o) const {
      return x < o.x || y < o.y || val < o.val;
  }
  test_struct_t(int a, int b, int c) : x(a), y(b), val(c) {}
};

The custom comparator I wrote is:

struct cmp_by_value {
  bool operator()(const test_struct_t *a, const test_struct_t *b) const 
  {
      return *a < *b;
  }
};

Now, in my main method I do the following:

testmap.insert({new test_struct_t(0, 0, 2 ), 6});
testmap.insert({new test_struct_t(0, 1, 2 ), 6});
testmap.insert({new test_struct_t(1, 1, 0 ), 6});
testmap.insert({new test_struct_t(1, 2, 0 ), 6});
testmap.insert({new test_struct_t(2, 1, 0 ), 6});
testmap.insert({new test_struct_t(1, 2, 1 ), 6});
testmap.insert({new test_struct_t(1, 2, 0 ), 6});

And then I print the values in the map with some format method:

std::string format(test_struct_t *e) {
    std::stringstream ss;
    ss << "(" << e->x << "," << e->y << ") = " << e->val;
    return ss.str();
}

And in the main:

std::map<test_struct_t*, unsigned int>::iterator it;
  for (it = testmap.begin(); it != testmap.end(); ++it) {
      std::cout << format(it->first) << std::endl;
 }

My program gives the following output:

(1,1) = 0
(1,2) = 0
(1,2) = 1
(2,1) = 0
(1,2) = 0
(0,0) = 2
(0,1) = 2

Can anyone explain to me why there is the duplicate entry "(1,2) k =0"? When I remove one of the other inserted statements, everything is fine and there is no duplicate. Any ideas?

EDIT: What I also found very interesting is that:

testmap[new test_struct_t(0, 0, 2 )] = 6;
testmap[new test_struct_t(0, 1, 2 )] = 6;
testmap[new test_struct_t(1, 1, 0 )] = 6;
testmap[new test_struct_t(1, 2, 0 )] = 6;
testmap[new test_struct_t(2, 1, 0 )] = 6;
testmap[new test_struct_t(1, 2, 1 )] = 6;
testmap[new test_struct_t(1, 2, 0 )] = 6;

Gives this:

(0,0) = 2
(1,1) = 0
(1,2) = 0
(0,1) = 2

Answer 1

Your operator< does not define a strict weak ordering. Try

bool operator<(const test_struct_t &o) const {
   return std::tie(x,y,val)<std::tie(o.x,o.y,o.val);
}