ordered set c++

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;

typedef tree<int, null_type, less<int>, rb_tree_tag,
                    tree_order_statistics_node_update>
                    ordered_set;

ordered_set ord_set;

int a;
ord_set.insert(a);
*ord_set.find_by_order(a);
ord_set.order_of_key(a);

#include <iostream> 
#include <set> 
#include <iterator> 
  
using namespace std; 
  
int main() 
{ 
    // empty set container 
    set <int, greater <int> > gquiz1;         
  
    // insert elements in random order 
    gquiz1.insert(40); 
    gquiz1.insert(30); 
    gquiz1.insert(60); 
    gquiz1.insert(20); 
    gquiz1.insert(50); 
    gquiz1.insert(50); // only one 50 will be added to the set 
    gquiz1.insert(10); 
  
    // printing set gquiz1 
    set <int, greater <int> > :: iterator itr; 
    cout << "\nThe set gquiz1 is : "; 
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) 
    { 
        cout << '\t' << *itr; 
    } 
    cout << endl; 
  
    // assigning the elements from gquiz1 to gquiz2 
    set <int> gquiz2(gquiz1.begin(), gquiz1.end()); 
  
    // print all elements of the set gquiz2 
    cout << "\nThe set gquiz2 after assign from gquiz1 is : "; 
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) 
    { 
        cout << '\t' << *itr; 
    } 
    cout << endl; 
  
    // remove all elements up to 30 in gquiz2 
    cout << "\ngquiz2 after removal of elements less than 30 : "; 
    gquiz2.erase(gquiz2.begin(), gquiz2.find(30)); 
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) 
    { 
        cout << '\t' << *itr; 
    } 
  
    // remove element with value 50 in gquiz2 
    int num; 
    num = gquiz2.erase (50); 
    cout << "\ngquiz2.erase(50) : "; 
    cout << num << " removed \t" ; 
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) 
    { 
        cout << '\t' << *itr; 
    } 
  
    cout << endl; 
  
    //lower bound and upper bound for set gquiz1 
    cout << "gquiz1.lower_bound(40) : "
         << *gquiz1.lower_bound(40) << endl; 
    cout << "gquiz1.upper_bound(40) : "
         << *gquiz1.upper_bound(40) << endl; 
  
    //lower bound and upper bound for set gquiz2 
    cout << "gquiz2.lower_bound(40) : "
         << *gquiz2.lower_bound(40) << endl; 
    cout << "gquiz2.upper_bound(40) : "
         << *gquiz2.upper_bound(40) << endl; 
  
    return 0; 
  
} 

#include <set>

set<type> name;  //set<int> s;

tree < int ,  null_type ,  less ,  rb_tree_tag ,  tree_order_statistics_node_update >

using namespace __gnu_pbds;

// constructing sets
#include <iostream>
#include <set>

bool fncomp (int lhs, int rhs) {return lhs<rhs;}

struct classcomp {
  bool operator() (const int& lhs, const int& rhs) const
  {return lhs<rhs;}
};

int main ()
{
  std::set<int> first;                           // empty set of ints

  int myints[]= {10,20,30,40,50};
  std::set<int> second (myints,myints+5);        // range

  std::set<int> third (second);                  // a copy of second

  std::set<int> fourth (second.begin(), second.end());  // iterator ctor.

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

  bool(*fn_pt)(int,int) = fncomp;
  std::set<int,bool(*)(int,int)> sixth (fn_pt);  // function pointer as Compare

  return 0;
}