Thinking caused by the error ＜ note: candidate expectations 1 argument, 0 provided ＞

Recently brushing PAT During the class a tutorial , Encountered an error as shown in the title . Topic link ：1561. PAT evaluating - AcWing Question bank

The code that causes the error is as follows ：

#include<bits/stdc++.h>
using namespace std;

int N,M;
// const int K = 6;


int n, k, m;
int P[K];

int K = 6;
struct stu
{
    string id;
    int grade[K];

    int total = 0;
    int count;

    //  stu()
    //  {
    //      cout << " The default constructor executes " << endl;
    //  }



    stu(const stu& oth)
    {
        this->id = oth.id;
        memcpy(this->grade, oth.grade, 4*k);
        this->total = oth.total;
        this->count = oth.count;
        cout << " Custom copy constructor execution " << endl;
    }

    stu(string _id):id(_id)
    {
        for(int i = 1; i <= k; ++i)
            grade[i] = -2;
        total = count = 0;
        cout << " Custom constructor execution " << endl;
    }

    stu& operator = (const stu& oth)
    {
        this->id = oth.id;
        // this->grade = oth.grade;
        memcpy(this->grade, oth.grade, 4*k);
        this->total = oth.total;
        this->count = oth.count;
        return *this;

        cout << " Overloaded assignment operators execute " << endl;
    }


    void calc()
    {
        for(int i = 1; i <= k; ++i)
        {
            total += max(0, grade[i]);
            count += grade[i] == P[i];
        }
    }

    bool has_submit()
    {
        for(int i = 1; i <= k; ++i)
        {
            if(grade[i] >= 0)
                return true;
        }
        return false;
    }

    bool operator < (const stu& oth)
    {
        if(this->total != oth.total)
            return this->total > oth.total;
        else if(this->count != oth.count)
            return this->count > oth.count;
        else
            return this->id < oth.id;

    }
};




int main()
{

    unordered_map<string, stu> students;

    cin >> n >> k >> m;
    for(int i = 1; i <= k; ++i) cin >> P[i];
    while(m--)
    {
        string u_id;
        int p_id, grade;
        cin >> u_id >> p_id >> grade;

        if(students.count(u_id) == 0)
            students[u_id] = stu(u_id); // Where something went wrong 
        students[u_id].grade[p_id] = max(students[u_id].grade[p_id],
        grade);
    }

    vector<stu> res;
    for(auto& [fi, se]: students)
    {
        if(se.has_submit())
        {
            se.calc();
            res.push_back(se);
        }
    }
    sort(res.begin(), res.end());

    for(int i = 0, rank=1; i < res.size(); ++i)
    {
        if(i && res[i].total != res[i-1].total)
            rank = i+1;
        cout << rank << " " << res[i].id << " " << res[i].total << " ";
        for(int j=1; j <= k; ++j)
        {
            if(res[i].grade[j] == -2)
                cout << "-"  ;
            else
                cout << max(res[i].grade[j], 0) ;

            if(j < k)
                cout << " ";
        }
        cout << endl;
    }

    return 0;
}

What caused the mistake ：

  Logically speaking , In the above code, I have met C++ Three five rule of ： Completed the custom implementation of the copy constructor , It's still wrong . Finally, after debugging, it is found that , The problem is that there is no default constructor .

The following test code is OK ：

#include <bits/stdc++.h>
using namespace std;

const int K = 6;
const int k = 5;
const int P[1000] = {0};
struct stu
{
    string id;
    int grade[K];

    int total = 0;
    int count;

    // stu()
    // {
    //     cout << " The default constructor executes " << endl;
    // }

    stu(const stu &oth)
    {
        this->id = oth.id;
        memcpy(this->grade, oth.grade, 4 * k);
        this->total = oth.total;
        this->count = oth.count;
        cout << " Custom copy constructor execution " << endl;
    }

    stu(string _id) : id(_id)
    {
        for (int i = 1; i <= k; ++i)
            grade[i] = -2;
        total = count = 0;
        cout << " Custom constructor execution " << endl;
    }

    stu &operator=(const stu &oth)
    {
        this->id = oth.id;
        // this->grade = oth.grade;
        memcpy(this->grade, oth.grade, 4 * k);
        this->total = oth.total;
        this->count = oth.count;
        return *this;

        cout << " Overloaded assignment operators execute " << endl;
    }

    void calc()
    {
        for (int i = 1; i <= k; ++i)
        {
            total += max(0, grade[i]);
            count += grade[i] == P[i];
        }
    }

    bool has_submit()
    {
        for (int i = 1; i <= k; ++i)
        {
            if (grade[i] >= 0)
                return true;
        }
        return false;
    }

    bool operator<(const stu &oth)
    {
        if (this->total != oth.total)
            return this->total > oth.total;
        else if (this->count != oth.count)
            return this->count > oth.count;
        else
            return this->id < oth.id;
    }
};

int main()
{
    stu A("AAA");

    stu B(A);
    stu C = A;

    return 0;
}

Again, there is no default constructor , But the test code will work . Therefore, it is preliminarily inferred that when a class object is used as the value of the hash table , This class must have a default parameterless constructor . After debugging, it is found that ：

1、 Whether it's Windows Your compiler is still linux The compiler , When a class object is used as the value of a hash table , When it is initialized with another object of the class , This class calls the default parameterless constructor first, and then calls the overloaded assignment operator , This may have something to do with the underlying implementation of the hash table .

2、 When the instance object of this class is not used as the value of the hash table , Use an existing instance to initialize a new instance of the same kind , Whether through stu B(A) Or through stu B = A All the methods are direct copy constructor methods .

So in the development process, even if the class overloads multiple constructors , It is better to keep the default constructor , In multi person collaborative development, it is not certain that others will take your class as the value of the hash table . Or assign default values to all parameters , This is also a feasible solution .

in addition , The above code can not pass the original question , You need to comment out the copy constructor and overloaded copy operator in the above , The reason should be with memcpy Copy grade This piece of code has something to do with .