Find the element in a std::set in C++11

Question

What would be the key in case of Objects being inserted into std::set? As for the following example, I am inserting objects of class Classcomp into the std::set. However, I want to find whether an object of Classcomp with a particular 'id = 1' exists in the std::set or not?

#include <iostream>
#include <set>

struct Classcomp {
  int val = 0;
  int id = 0; ///ID for the object
  bool operator() (const Classcomp& lhs, const Classcomp& rhs) const
  {return lhs.val<rhs.val;}
};

int main ()
{

  Classcomp c1,c2,c3,c4;
  c1.val = 92;c1.id = 2;
  c2.val = 94;c2.id = 3;
  c3.val = 10;c3.id = 1;

  std::set<Classcomp,Classcomp> fifth;                 // class as Compare

    fifth.insert(c1);fifth.insert(c2);fifth.insert(c3);

    for (auto x: fifth) {std::cout << x.id << "  " << x.val << std::endl;} 

    if (fifth.find()) {std::cout << "Got it";} //What should I pass as arguments to fifth.find()?

  return 0;
}

Answer 1

Sets is different from the map by exactly the fact that values of objects inside the set are their keys . If you need to separate a key from the value, you need a key-value container, such as std::map or std::unordered_map .

Clarification I am not talking about the option of simply iterating over all objects in the set and checking every object to have specified key - which is easily done with std::find . If that's what you want, please clarify.

Answer 2

As your class is sorted by field val you have following options:

• iterate over all elements and find one with particular id by std::find or std::find_if
• create another container and sort there by id (maybe using std::share_ptr to avoid duplicates)
• use boost::multiindex and create index by val as well as id