C++ Changing a set default order based of a parameter

Question

I'm trying to modify a functor to accept an argument so I could sort a set based in that parameter. I just can't figure it out

struct sortset {
  bool operator() (const string& s1, const string& s2) const {
    //Sorting algorithm depending on "sortingVar"
  }
};

main() {
  string sortingVar; cin >> sortingVar;
  set<string, sortset> mySet;
  ...
}

What type of parameter you want? Also you are missing parenthesis set in your functor; bool operator`()` (const string& s1, const string& s2) const — Onur A., Mar 10 '18 at 21:16
Possible duplicate of [Inserting into unordered\_set with custom hash function](https://stackoverflow.com/questions/15869066/inserting-into-unordered-set-with-custom-hash-function) — Smit Ycyken, Mar 10 '18 at 21:20
@JoanPastor is this string different from the ones you store in your set? — Onur A., Mar 10 '18 at 21:22

Lightness Races in Orbit · Accepted Answer · 2018-03-10T21:39:26.583

Give the comparator a constructor which accepts the argument to determine the comparator's behaviour:

struct sortset
{
  sortset(const string& sortingVar) : sortingVar(sortingVar) {}

  bool operator() (const string& s1, const string& s2) const {
    //Sorting algorithm based on "sortingVar"
  }
private:
  const string sortingVar;
};

int main() {
  string sortingVar; cin >> sortingVar;

  // Either:
  set<string, sortset> mySet{sortset{sortingVar}};

  // Or:
  sortset comp{sortingVar};
  set<string, sortset> mySet{comp};

  // ...
}

Notice that the set constructor allows you to provide an actual instance of the comparator, so you can use that to pass one that is not default-constructed (a good thing too, since no such thing can now exist).

Add some std::moves if you like.

Don't forget to give main a return type.

C++ Changing a set default order based of a parameter

1 Answers1