VISUAL C++定義類

Question

的排序列表我有一個類的學生，在class_student.h定義如下：

#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; 
    } 

}; 

我在另一頭文件中定義一個列表List<student^>^ students = gcnew List<student^>();（與包括類文件）。該列表中添加了幾個班學生的實例。

我想用students->Sort()或類似的東西來排序這個列表。我嘗試覆蓋operator<，無論是在課程定義中還是在課程外部，但是當我撥打Sort()時，仍然收到一條錯誤消息，指出無法比較項目。

當試圖通過operator<要做到這一點，我用這個，如果這能幫助：

#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; 
    } 
}; 

或者這樣：

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

作爲評論者指出，這是C++/CLI，而不是C++。如果你想編寫C++，我建議你創建一個真正的C++項目。如果你想編寫託管代碼，我建議你創建一個C＃項目。 C++/CLI旨在用作託管語言（如C＃）和C++之間的橋樑，並且它通常不應用於主應用程序開發。

您需要實現接口IComparable<student^>^，而不是實現運算符。

在.NET中，運營商不能在一個指定的接口，因此IComparable<T>接口指定方法CompareTo(T)，它返回-1如果this < other，0如果相等，並1如果更大。

爲保持一致性和最佳實踐，您還應該覆蓋equals和hashCode，並執行IEquatable<T>。