技術標籤：C++

在構造map、set、multimap、multiset這些序列式容器時，預設按照key值來進行排序，比較器預設從小到大。在構造時加上greater<T> 會讓容器的比較器按照key值從大到小排序，less<T>則相反。

#include<iostream>
#include<map>
#include<functional>
using namespace std;
int main() {
	map<int, string,greater<int>> fruits;
	fruits.
 
insert(pair<int, string>(1, "apple"));
	fruits.insert(pair<int, string>(2, "orange"));
	fruits.insert(pair<int, string>(4, "banana"));
	fruits.insert(pair<int, string>(3, "peach"));
	for (auto i : fruits) {
		cout << i.first <<
 
 " " << i.second << endl;
	}
	return 0;

}

// STRUCT TEMPLATE greater
template <class _Ty = void>
struct greater {
     typedef _Ty first_argument_type;
     typedef _Ty second_argument_type;
     typedef bool result_type;

    constexpr bool operator()(const _Ty& _Left,
 
 const _Ty& _Right) const {
        return _Left > _Right;
    }
};

// STRUCT TEMPLATE less
template <class _Ty = void>
struct less {
     typedef _Ty first_argument_type;
     typedef _Ty second_argument_type;
     typedef bool result_type;

    constexpr bool operator()(const _Ty& _Left, const _Ty& _Right) const {
        return _Left < _Right;
    }
};```