• 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