Question

我有一个班级学生，在class_student.h中定义如下：

#pragma once
using namespace System;
using System::Windows::Forms::MessageBox;
using namespace System::Collections::Generic;
using namespace MySql::Data::MySqlClient;

public ref class student {

public:
    String^ user_name;
    String^ my_class;
    int weighted_grades;
    int weighted_totals;
    float average_percent;

    int get_weighted_grades() {
        String^ con_string = L"datasource=127.0.0.1;port=3306;username=root;password=1234;database=comp4;";
        MySqlConnection^ con_database = gcnew MySqlConnection(con_string);
        MySqlCommand^ get_grades = gcnew MySqlCommand("select coalesce(sum(method_marks) + sum(accuracy_marks), 0) from tbl_grades inner join tbl_test_instances on tbl_grades.test_instance=tbl_test_instances.id inner join tbl_students on tbl_test_instances.student_id = tbl_students.username inner join tbl_tests on tbl_test_instances.test_id=tbl_tests.test_id where tbl_students.class = tbl_tests.class and username='" + user_name + "';", con_database);
        MySqlDataReader^ my_reader;
        int grades;
        try {
            con_database->Open();
            my_reader = get_grades->ExecuteReader();
            my_reader->Read();
            grades = my_reader->GetInt32(0);
        }
        catch (Exception^ ex) {
            MessageBox::Show(ex->Message);
        }
        return grades;
    }

    int get_weighted_totals() {
        String^ con_string = L"datasource=127.0.0.1;port=3306;username=root;password=1234;database=comp4;";
        MySqlConnection^ con_database = gcnew MySqlConnection(con_string);
        MySqlCommand^ get_totals = gcnew MySqlCommand("select coalesce(sum(method_marks) + sum(accuracy_marks), 0) from tbl_questions inner join tbl_test_questions on tbl_questions.question_id=tbl_test_questions.question_id inner join tbl_test_instances on tbl_test_questions.test_id=tbl_test_instances.test_id inner join tbl_students on tbl_test_instances.student_id=tbl_students.username inner join tbl_tests on tbl_test_instances.test_id=tbl_test_questions.test_id where tbl_students.class=tbl_tests.class and username='" + user_name + "';", con_database);
        MySqlDataReader^ my_reader;
        int totals;
        try {
            con_database->Open();
            my_reader = get_totals->ExecuteReader();
            my_reader->Read();
            totals = my_reader->GetInt32(0);
        }
        catch (Exception^ ex) {
            MessageBox::Show(ex->Message);
        }
        return totals;
    }

    student(String^ user_name, String^ txt_class) {
        this->user_name = user_name;
        this->my_class = txt_class;
        this->weighted_grades = get_weighted_grades();
        this->weighted_totals = get_weighted_totals();
        this->average_percent = ((float)weighted_grades / (float)weighted_totals) * 100;
    }

};

我在另一个头文件中定义了一个列表List<student^>^ students = gcnew List<student^>();（包含了类文件）。此列表中添加了几个班级学生的实例。

我想使用students->Sort()或类似的东西来排序这个列表。我尝试覆盖operator<，在类定义内部和外部，但是当我调用Sort()时，我仍然收到一条错误消息，指出无法比较这些项目。

当试图通过operator<执行此操作时，我使用了这个，如果有帮助：

#pragma once
using namespace System;
using System::Windows::Forms::MessageBox;
using namespace System::Collections::Generic;
using namespace MySql::Data::MySqlClient;

public ref class student {

public:
    String^ user_name;
    String^ my_class;
    int weighted_grades;
    int weighted_totals;
    float average_percent;

    int get_weighted_grades() {
        String^ con_string = L"datasource=127.0.0.1;port=3306;username=root;password=1234;database=comp4;";
        MySqlConnection^ con_database = gcnew MySqlConnection(con_string);
        MySqlCommand^ get_grades = gcnew MySqlCommand("select coalesce(sum(method_marks) + sum(accuracy_marks), 0) from tbl_grades inner join tbl_test_instances on tbl_grades.test_instance=tbl_test_instances.id inner join tbl_students on tbl_test_instances.student_id = tbl_students.username inner join tbl_tests on tbl_test_instances.test_id=tbl_tests.test_id where tbl_students.class = tbl_tests.class and username='" + user_name + "';", con_database);
        MySqlDataReader^ my_reader;
        int grades;
        try {
            con_database->Open();
            my_reader = get_grades->ExecuteReader();
            my_reader->Read();
            grades = my_reader->GetInt32(0);
        }
        catch (Exception^ ex) {
            MessageBox::Show(ex->Message);
        }
        return grades;
    }

    int get_weighted_totals() {
        String^ con_string = L"datasource=127.0.0.1;port=3306;username=root;password=1234;database=comp4;";
        MySqlConnection^ con_database = gcnew MySqlConnection(con_string);
        MySqlCommand^ get_totals = gcnew MySqlCommand("select coalesce(sum(method_marks) + sum(accuracy_marks), 0) from tbl_questions inner join tbl_test_questions on tbl_questions.question_id=tbl_test_questions.question_id inner join tbl_test_instances on tbl_test_questions.test_id=tbl_test_instances.test_id inner join tbl_students on tbl_test_instances.student_id=tbl_students.username inner join tbl_tests on tbl_test_instances.test_id=tbl_test_questions.test_id where tbl_students.class=tbl_tests.class and username='" + user_name + "';", con_database);
        MySqlDataReader^ my_reader;
        int totals;
        try {
            con_database->Open();
            my_reader = get_totals->ExecuteReader();
            my_reader->Read();
            totals = my_reader->GetInt32(0);
        }
        catch (Exception^ ex) {
            MessageBox::Show(ex->Message);
        }
        return totals;
    }

    student(String^ user_name, String^ txt_class) {
        this->user_name = user_name;
        this->my_class = txt_class;
        this->weighted_grades = get_weighted_grades();
        this->weighted_totals = get_weighted_totals();
        this->average_percent = ((float)weighted_grades / (float)weighted_totals) * 100;
    }
    bool operator<(student^ other) {
        return this->average_percent > other->average_percent;
    }
};

或者这个：

class_student.h：

[class definition as shown at the top]

bool operator<(student^ s1, student^ s2);

class_student.cpp：

#include "class_student.h"

bool operator<(student^ s1, student^ s2) {
    return s1->average_percent > s2->average_percent;
}

Answer 1

正如评论者所说，这是C ++ / CLI，而不是C ++。如果你想编写C ++，我建议你创建一个实际的C ++项目。如果要编写托管代码，我建议您创建一个C＃项目。 C ++ / CLI旨在充当托管语言（如C＃）和C ++之间的桥梁，通常不应用于主应用程序开发。

您需要实现接口IComparable<student^>^，而不是实现运算符。

在.Net中，不能在接口中指定运算符，因此IComparable<T>接口指定方法CompareTo(T)，如果-1，则返回this < other，{{1}如果相等，则0如果更大。

为了保持一致性和最佳做法，您还应覆盖1和equals，并实施hashCode。

visual c ++自定义类的排序列表

1 个答案: