CODEWITHSUNDEEP
c++c++11stl
Uddhav Mishra
Uddhav Mishra

Reputation: 76

Using set of function c++

I am trying to create a set of function pointers in c++ but getting error while inserting / deleting elements from it. 

#include<bits/stdc++.h>
using namespace std;

void func(int x) {
  cout<<x;
}

int main() {
  set <function<void (int)>> mine;
  mine.insert(func);
  return 0;
}

i am getting error /usr/include/c++/6/bits/stl_function.h:386:20: error: no match for ‘operator<’ (operand types are ‘const std::function’ and ‘const std::function’). I think that this problem is because of operator that will be used to compare set values , can someone suggest how to make this work ? How can i write a comparator for function pointers in this case ?

Upvotes: 0

Views: 1247

Answers (1)

Max Langhof
Max Langhof

Reputation: 23701

If you want to store just function pointers, you don't need std::function:

using MyFunctionPointer = void(*)(int);

void func(int x);

std::set<MyFunctionPointer> orderedSet;

int main()
{
    orderedSet.emplace(func);
}

Demo

This works because you can compare (for std::set) or hash (for std::unordered set) function pointer values. But comparing or hashing std::function instances is not implemented in the standard library, and there is no portable way to add that after the fact.

Edit: As pointed out by @HolyBlackCat, while the builtin operator< is not required to induce the required total order on function pointers, std::less (as used by std::set) is required to do so for any pointer.

Upvotes: 3

Related Questions