我正在尝试将一组对值插入 c++11 中的 std::map
中。但是,这些值似乎没有插入到 std::map
中。请仔细检查我的代码。我感谢任何和所有的帮助。
#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<map>
#include<cstdlib>
#include<utility>
#include<ctime>
#include "print.h"
class ReportCard
{
private:
std::map<std::string, double> m_report_card;
public:
std::map<std::string, double> getReportCardInstance() { return m_report_card; }
};
class Student
{
private:
int m_roll_no;
std::string m_name;
ReportCard m_reportCard;
public:
Student(int inRollNo, const std::string& inName) :
m_roll_no(inRollNo), m_name(inName)
{}
std::string getName() { return m_name; }
int getRollNo() { return m_roll_no; }
ReportCard getReportCard() { return self.m_reportCard; }
int getReportCardSize() { return m_reportCard.getReportCardInstance().size(); }
};
class Driver
{
private:
std::vector<Student> student_list;
std::vector<Student> temp;
public:
void studentTestPopulate()
{
student_list.push_back(Student(1, "Tim"));
student_list.push_back(Student(2, "Matt"));
student_list.push_back(Student(100, "Luke"));
student_list.push_back(Student(68, "Lissy"));
student_list.push_back(Student(20, "Tony"));
student_list.push_back(Student(33, "Joseph"));
student_list.push_back(Student(14, "Sid"));
student_list.push_back(Student(15, "Roby"));
student_list.push_back(Student(44, "Rohan"));
student_list.push_back(Student(11, "Kevin"));
student_list.push_back(Student(19, "George"));
}
void reportCardPopulate()
{
for (auto& student : student_list)
{
std::cout << student.getName() << std::endl;
student.getReportCard().getReportCardInstance().insert(std::make_pair<std::string, double>("Math", generateMark));
//This is the function that does not work. No marks are printed!!
for (auto& mark : student.getReportCard().getReportCardInstance())
{
std::cout << mark.first << " " << mark.second;
}
//student.getReportCard().getReportCardInstance().insert(std::make_pair("Science", generateMark));
//student.getReportCard().getReportCardInstance().insert(std::make_pair("Geography", generateMark));
//student.getReportCard().getReportCardInstance().insert(std::make_pair("French", generateMark));
//student.getReportCard().getReportCardInstance().insert(std::make_pair("History", generateMark));
}
}
void showAllStudentDetails()
{
for (auto& student : student_list)
{
std::cout << student.getName() << std::endl;
std::cout << student.getRollNo() << std::endl;
std::cout << "REPORT CARD : " << student.getReportCardSize() << std::endl << std::endl;
for (auto& mark : student.getReportCard().getReportCardInstance())
{
std::cout << mark.first << std::endl;
std::cout << mark.second << std::endl;
}
}
}
};
int main()
{
srand(time(NULL));
Driver driver;
driver.studentTestPopulate();
driver.reportCardPopulate();
//driver.showAllStudentDetails();
}
reportCardPopulate()
函数应该将值对插入到report_card 映射中。但是,插入功能似乎不起作用。
当我们尝试在 reportCardPopulate()
函数中打印值时,它不会打印任何内容。当我尝试打印 map 大小时,它打印0
。当我使用 sizeof() 打印尺寸时,它在插入之前和之后打印相同的尺寸。
最佳答案
以下功能
std::map<std::string, double> getReportCardInstance() { ... }
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ReportCard getReportCard() { ... }
//^^^^^^^^
返回 std::map<std::string, double>
的拷贝和ReportCard
分别类。因此,无论您在此处插入什么内容
student.getReportCard().getReportCardInstance().insert(std::make_pair<std::string, double>("Math", generateMark));
在上述拷贝上执行,因此原始成员位于 ReportCard
中(即 m_report_card
)永远不会被更新。在调用上述行之后,拷贝将被销毁,期望它起作用是没有意义的。
其次,显示的代码是错误的,因为在 C++ 中你应该使用 this
不是self
ReportCard getReportCard()
{
return self.m_reportCard;
//^^^^ --> should be `return this->m_reportCard;`
// or simply `return m_reportCard;`
}
更正上述内容并通过引用返回成员将使代码正常工作。 ( See live online )
std::map<std::string, double>& getReportCardInstance()
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^
{
return m_report_card;
}
ReportCard& getReportCard()
//^^^^^^^^
{
return m_reportCard;
}
话虽这么说,你的ReportCard
和Student
如果执行上述操作,类将公开成员。这不是一个好的设计。如果这些仅用于 Driver
的内部使用类,你可以将它们保留为 private
Driver
的属性类。
#include <vector>
#include <map>
#include <string>
#include <iostream>
class Driver /* final */ // -> optional
{
private: // Student is private for Driver class
class Student
{
// type alias is enough for the map
using ReportCard = std::map<std::string, double>;
private:
int m_roll_no;
std::string m_name;
ReportCard m_reportCard;
public:
Student(int inRollNo, const std::string& inName)
: m_roll_no{ inRollNo }, m_name{ inName }
{}
// make the member functions const if they are not modifing the members
const std::string& getName() const { return m_name; }
int getRollNo() const { return m_roll_no; }
ReportCard& getReportCard() { return m_reportCard; }
std::size_t getReportCardSize() const { return m_reportCard.size(); }
};
private:
std::vector<Student> student_list;
std::vector<Student> temp;
public:
void studentTestPopulate()
{
// construct the `Student` in-place using `std::vector::emplace_back`
student_list.emplace_back(1, "Tim");
student_list.emplace_back(2, "Matt");
student_list.emplace_back(100, "Luke");
student_list.emplace_back(68, "Lissy");
student_list.emplace_back(20, "Tony");
student_list.emplace_back(33, "Joseph");
student_list.emplace_back(14, "Sid");
student_list.emplace_back(15, "Roby");
student_list.emplace_back(44, "Rohan");
student_list.emplace_back(11, "Kevin");
student_list.emplace_back(19, "George");
}
void reportCardPopulate()
{
for (auto& student : student_list)
{
std::cout << student.getName() << "\n";
student.getReportCard().emplace(student.getName(), 12.0);
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// use `std::map::emplace` for constructing `ReportCard` in-place
for (auto& mark : student.getReportCard())
{
std::cout << mark.first << " " << mark.second << "\n";
}
}
}
// ... other members
};
int main()
{
Driver driver;
driver.studentTestPopulate();
driver.reportCardPopulate();
}
关于c++ - 如何在 C++11 中插入 std::map ?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/57701329/