成绩排序问题出错了，怎么回事

#include<iostream>
#include<string>
using namespace std;

struct Student
{
    string m_name;
    int m_score;
};

void swap(Student& arr1,Student& arr2)
{
   Student temp;
   temp=arr1;
   arr1=arr2;
   arr2=temp;
}

int main()
{
    Student arr[20];
    int num,score;
    string name;
    cin >> num;
    for (int i = 0; i < num; i++)
    {
        cin >> name >> score;
        arr[i].m_name=name;
        arr[i].m_score=score;
    }
    for (int i = 0; i < num - 1; i++)
    {
        for (int j = 0; j < num - i - 1; j++)
        {
            if (arr[j].m_score < arr[j+1].m_score)
                swap(arr[j],arr[j+1]);
            if (arr[j].m_score == arr[j + 1].m_score)
            {
                if ((int)arr[j].m_name[0] > (int)arr[j + 1].m_name[0])
                    swap(arr[j],arr[j+1]);
            }
        }
    }
    for (int i = 0; i < num; i++)
    {
        cout << arr[i].m_name << " " << arr[i].m_score << endl;
    }
    return 0;
}

帮我看看哪里出错了

第二个for循环 j= i
for (int j = i; j < num - i - 1; j++);
string自带比较大小
if ((int)arr[j].m_name[0] > (int)arr[j + 1].m_name[0])改为if (arr[j].m_name > (int)arr[j + 1].m_name)