C++类对象排序operator重载操作

• 类内默认含有this指针，bool operator==(const T& a)
• 类外则需要写两个参数，bool operator==(const T& a, const T& b)

class People
{
public:
    string name;
    int id;
    People(string n, int i):name(n),id(i){}
    
    bool operator==(const People& a)
    {
        return id == a.id && name==a.name;
    }
    
    bool operator<( const People& a)
    {
        if(id == a.id)
            return name < a.name;
        return id < a.id;
    }
} ;

bool operator==(const People& a, const People& b)
{
    return a.id == b.id && a.name==b.name;
}

bool operator<(const People &a, const People& b)
{
    if(a.id == b.id)
        return a.name < b.name;
    return a.id < b.id;
}

int main()
{
    vector<People> vec;
    vec.push_back(People("Michael",23));
    vec.push_back(People("James",23));
    vec.push_back(People("Kobe",24));
    vec.push_back(People("James",23));

	cout << "-----按id, then name排序----" << endl;
    sort(vec.begin(), vec.end()); // 调用operator<
    for(auto& v : vec)
        cout << v.id << " " << v.name << endl;
    cout << "------去重------------------" << endl;
    vec.erase(unique(vec.begin(), vec.end()),vec.end());
    // unique删除相邻的一样的，要先排序，unique调用 operator==
    for(auto& v : vec)
        cout << v.id << " " << v.name << endl;
    cout << "-----按名称排序-------------" << endl;
    sort(vec.begin(), vec.end(),[](People& a, People& b)
    {
        return a.name < b.name;
    });
    for(auto& v : vec)
        cout << v.id << " " << v.name << endl;
    return 0;
}

运行结果：
-----按id, then name排序----
23 James
23 James
23 Michael
24 Kobe
------去重------------------
23 James
23 Michael
24 Kobe
-----按名称排序-------------
23 James
24 Kobe
23 Michael