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

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);
  }
  ...
};