C++ 仿函数行为

Question

#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;


struct cmp {
        bool operator()(const int& i, const int&  j) const{
            return false;
        }   
} ;

struct cmp2 {
        bool operator()(const int& i, const int&  j) const{
            return false;
        }   
} cmp2_item;


class Solution {
public:

    vector<int> smth(vector<int> arr, int k) {
        // nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp); #ERROR
        // nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp()); #WORKS
        // nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp2_item); # WORKS
        // sort(arr.begin(), arr.end(), cmp); #ERROR
        // sort(arr.begin(), arr.end(), cmp()); #WORKS
        // set<int, cmp> s; # WORKS
        // set<int, cmp2_item> s; # ERROR
        return {};
    }
};

int main() {
    // your code goes here
    Solution s;
    s.smth({}, 1);
    return 0;
}

I want to understand why this code behaves in this way.

• for nth_element we expect a functor so it makes sense to include like cmp(), but why does it start working without the (), when i instantiate the struct and use that?
• Similar in sort
• while when using it for a comparator for a set, it only works if the struct is not instantiated and without the ()

Can someone please clarify using the signatures why this is so?

nth_element : template< class RandomIt, class Compare > void nth_element ( RandomIt first, RandomIt nth, RandomIt last, Compare comp );

sort

  void sort (RandomAccessIterator first, RandomAccessIterator last, Compare comp); 

set :

           class Compare = less<T>,        // set::key_compare/value_compare
           class Alloc = allocator<T>      // set::allocator_type
           > class set;

Answer 1

One part of this is that cmp2_item is not a type, its an instance of the type cmp2. So you can't pass that as a class type. You might be able to do:

    set<int, cmp> s; //# WORKS
    set<int, decltype(cmp2_item)> s2; //# NOW WORKS

For these:

    // Not ok because cmp is not a function comparison object, its a type
    nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp); #ERROR
    // Ok because this is effectively passing a functor (and not just a type) 
    // I believe this takes a copy of the functor type (by construction), I 
    // am not quite so clear on the inner workings of the compiler in this
    // case. I guess its by type comparison, but possible compiler
    // implementation specific?
    nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp()); #WORKS
    // This is ok because it passes an actual comparison object (that has
    // operator () defined).
    nth_element(arr.begin(), arr.begin()+k, arr.end(), cmp2_item); # WORKS

Basically you have to look more closely at what you are passing: a type, an object or a function - and what the specific STL accepts as a parameter.

Note:

comparison function object (ie an object that satisfies the requirements of Compare) which returns true if the first argument is less than (ie is ordered before) the second.

See here: enter link description here