The minimal example of the problem I'm having is reproduced below:

```
#include <set>
using namespace std;

class foo {
public:
  int value, x;
  foo(const int & in_v) {
   value = in_v;
   x = 0;
  }
  bool operator<(const foo & rhs) const {
   return value < rhs.value; 
 }
};

int main() {
  foo y(3);
  set<foo> F;
  F.insert(y);

  // Now try to modify a member of the set
  F.begin()->x=1;
  return 0;
}

```

With the error `error: assignment of data-member ‘foo::value’ in read-only structure`. I feel like I'm missing something simple here, but why am I unable to modify the member `x` in my class?