c++ - 此示例代码无法编译

Question

我无法理解在 cygwin shell 中编译此代码时收到的错误消息。该消息很长，但在这 1,000 行错误的中间某处显示:

no matching call for operator <

这是什么意思？这是我的代码:

#include <iostream>
#include <string>
#include <set>
#include <algorithm>
#include <iterator>

using namespace std;

struct Grade{
 string id;
 int score;

  bool operator() (Grade& a, Grade& b){
        return a.id < b.id;   
    } 
};  

int main()
{   
    Grade g;
    set<Grade> gs;

    g.id = "ABC123";
    g.score = 99;
    gs.insert(g);

    g.id = "BCD321";
    g.score = 96;
    gs.insert(g);

    for(auto it : gs)
        cout << it.id << "," << it.score;

    return 0;
}

Answer 1

集合需要其元素类型来定义小于运算符。参见 http://www.cplusplus.com/reference/set/set/?kw=set

你可以这样定义它(在等级定义之后):

bool operator< (const Grade& a, const Grade& b){
    return a.id < b.id;
}