CODEWITHSUNDEEP
c++
The Philomath
The Philomath

Reputation: 992

Vector of maps with custom comparator

I am trying to create a vector of maps. Each map has a different comparator.

Here is what I have tried:-

#include<iostream>
#include<map>
#include<vector>

template <class T>
struct head {
  virtual bool operator() (const T& x, const T& y) const = 0;
};

template <class T>
struct greater: head<T> {
  bool operator() (const T& x, const T& y) const {return x>y;}
};

template <class T>
struct less: head<T> {
  bool operator() (const T& x, const T& y) const {return x<y;}
};

int main()
{
    std::vector<std::map<int, int, head<int>>> mp;
    return 0;
}

But I am getting an error that my operator() is pure virtual in head.

Please tell me what is the correct way to achieve this?

Upvotes: 1

Views: 176

Answers (1)

John Zwinck
John Zwinck

Reputation: 249133

You need a single comparator type that works for all your vectors. Like this:

template<typename T>
struct comp {
  comp(bool gt) : do_greater(gt) {} 
  bool operator() (const T& x, const T& y) const
  {
    return do_greater ? x > y : x < y;
  }
  bool do_greater;
};

int main()
{
  std::vector<std::map<int, int, comp<int>>> mp;
  mp.emplace_back(false); // first map uses less-than
  mp.emplace_back(true); // first map uses greater-than
}

That is, the choice of comparison function needs to be driven by the state initialized in each std::map constructor. That single ?: branch is probably better for performance than calling a virtual method every time, too.

Upvotes: 1

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