在集合中存储相等的对象

Question

我正在尝试做的事情：将set 中的元素按elem->value 排序，但仍然根据它们指向的内存地址识别它们是不同的元素。

#include <iostream>
#include <set>

using namespace std;

struct Node {
    int value;
    
    Node(int v) : value(v) {}
};

int main()
{
    Node* a = new Node(10);
    Node* b = new Node(10);
    Node* c = new Node(20);

    if (a == b) {
        cout << "a == b" << endl;
    }
    
    if (a == c) {
        cout << "a == c" << endl;
    }
    
    auto comparator = [](Node* lhs, Node* rhs) {
        return lhs->value < rhs->value;
    };

    set<Node*, decltype(comparator)> s(comparator);
    s.insert(a);
    s.insert(b);
    s.insert(c);
    
    cout << "after inserts, s.size() = " << s.size() << endl;
    
    s.erase(a);
    
    cout << "after erase, s.size() = " << s.size() << endl;
    
    delete(a);
    delete(b);
    delete(c);

    return 0;
}

由于a != b，我期望得到：

after inserts, s.size() = 3
after erase, s.size() = 2

但相反，我得到：

after inserts, s.size() = 2
after erase, s.size() = 1

使用lhs->value <= rhs->value，我得到：

after inserts, s.size() = 3
after erase, s.size() = 3

使用multiset，我得到：

after inserts, s.size() = 3
after erase, s.size() = 1

Answer 1

你应该使用内存地址进行抢七。

例子：

auto comparator = [](Node* lhs, Node* rhs) {
    return lhs->value < rhs->value ||
        (lhs->value == rhs->value && lhs < rhs);
};