1. Common sort

Example from cplusplus.com:

// sort algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::sort
#include <vector>       // std::vector

bool myfunction (int i,int j) { return (i<j); }

struct myclass {
  bool operator() (int i,int j) { return (i<j);}
} myobject;

int main () {
  int myints[] = {32,71,12,45,26,80,53,33};
  std::vector<int> myvector (myints, myints+8);               // 32 71 12 45 26 80 53 33

  // using default comparison (operator <):
  std::sort (myvector.begin(), myvector.begin()+4);           //(12 32 45 71)26 80 53 33

  // using function as comp
  std::sort (myvector.begin()+4, myvector.end(), myfunction); // 12 32 45 71(26 33 53 80)

  // using object as comp
  std::sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)

  // print out content:
  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}
 

2. Passing extra parameters to STL sort

Refer to: https://varrunr.wordpress.com/2012/07/30/passing-extra-parameters-to-stl-sort/

//My vector looked like
class Topic;
class Query;
vector<Topic> list_topics;

//I needed to sort the vector list_topics using a Query object as reference. But traditionally, a comparator function takes in only 2 arguments of the same type as the elements that are being sorted.

bool compareTopics(const Topic t1 , const Topic t2){}; // Traditional comparator
sort( list_topics.begin() , list_topics.end() , compareTopics);

//I needed to pass in a third parameter to make it,
bool compareTopics(const Topic t1, const Topic t2, Query query){
// compare here
};

//In order to do this, we need to make use of functors i.e an object that can be called like a regular function. To implement this in C++, we need to define a class and implement to () operator function, while passing the extra arguments we need to the class constructor.

class TopicSorter{
    Query query_;
public:
    TopicSorter(Query query){ query_ = query; }
    bool operator()(Topic t1, Topic t2) const {
    return compareTopics( t1 , t2 , query_);
    }
};

//Now we can sort the vector of objects
Query query;
sort(list_topics.begin() , list_topics.end() , TopicSorter(query));