Using map with class error, compile error

Question

I have the following compiler error, how could I fix it?

error:  instantiated from `_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&a         


        

Answer 1

For a std::map you need to have overloaded operator< on the Key type of the map, because that is how the map will insert elements into it's underlying container.

class ar { 
  public:
  int a;
  int b;
  int c;
  public:
  ar() : a(0), b(0), c(0) {}
  bool operator<(const ar& other) const;
  };

bool ar::operator< (const ar& other) const // note the function has to be const!!!
{
   return (other.a < a) && (other.b < b) && (other.c < c); // or some such ordering
}

When overloading operator<, it's a good idea to in a similar fashion also overload operator>.

Answer 2

You need a comparison function for the map. You can either create operator< that compares two instances of ar, or you can create a custom function and pass it as the 3rd template parameter.

An example of the former might be:

class ar {
  ...
  bool operator<(const ar& rhs) const {
    return std::tie(a,b,c) < std::tie(rhs.a, rhs.b, rhs.c);
  }
  ...
};

Answer 3

operator < must be available for the key type, or you should provide a comparison functor to the map constructor.