创建基础 class 对象的向量并将派生的 class 对象存储在其中

Question

我正在尝试创建一个员工数据库（员工向量）。有 3 种类型的员工，即。 employees 是基础 class，Manager、Engg 和 Scientist 是派生的 class。每个员工都有名字和姓氏。除了名字之外，这 3 种类型的员工中的每一种都有独特的统计数据，即。经理有人数meetings/week 而工程师有工作经验等等。

我有几个问题 1. 我应该将派生对象向上转换为基数 class 还是将基数 class 向下转换为派生对象 class？ 2. 我如何使用多态性来覆盖方法，因为我希望用户添加员工类型，并根据所选的类型显示相应的输入字段，即。如果是经理，除了名字和姓氏之外，程序还应该要求 meetings/week?

这是我的 Class 文件

class Employee{
public:
Employee();
 Employee(string fName, string lName, int sal);
virtual void printEmp();
string getFirstName();
string getLastName();

protected:
string m_fName;
string m_lName;
int m_sal;
};


class Manager : public Employee{
public:
Manager();
Manager(string fName, string lName, int sal, int meets, int hols);
void printEmp();


protected:
int m_meets;
int m_hols;
};

这是实现

Employee::Employee(){
m_fName = "Default";
m_lName = "Default";
m_sal = 0;
}


Employee::Employee(string fName, string lName, int sal){
m_fName = fName;
m_lName = lName;
m_sal = sal;
}

void Employee::printEmp(){
cout << "First Name: " << m_fName << endl
    << "Last Name: " << m_lName << endl
    << "Salary: " << m_sal << endl;
}


string Employee::getLastName(){
return m_lName;
}

string Employee::getFirstName(){
return m_fName;
}

Manager::Manager(string fName, string lName, int sal, int meets, int hols) :         Employee(fName, lName, sal), m_meets(meets), m_hols(hols)
{
//empty
}

void Manager::printEmp(){
Employee::printEmp();
cout << "Meets/Week: " << m_meets << endl
    << "Holidays/Year: " << m_hols << endl << endl;

这是主要内容

int main(){

    bool exit = false;
    vector<Employee*> dBVector;

    while (!exit){

        cout << "Welcome to Employee Database, Enter an option to continue..." << endl;
        cout << "1) Add an Employee, 2) Delete an Employee, 3) Save Database, 4) Exit" << endl;
        int input;
        cin >> input;

        string fNameInp;
        string lNameInp;
        int salInp;
        string lNameSearch;
        int i; // for loop in Delete employee case
        bool deleted = false;

        switch (input){
        case 1: //Add
            cout << "1) Add a Manager, 2) Add an Engg, 3) Add a Researcher" << endl;
            int empInput;
            cin >> empInput;



            if (empInput == 1){


                cout << "Enter First Name: ";
                cin >> fNameInp;

                cout << "Enter Last Name: ";
                cin >> lNameInp;

                cout << "Enter Salary: ";
                cin >> salInp;

                cout << "Number of meetings/week: ";
                int meetsInp;
                cin >> meetsInp;

                cout << "Number of holidays/year: ";
                int holsInp;
                cin >> holsInp;

                Manager mEmp(fNameInp, lNameInp, salInp, meetsInp, holsInp);
                Employee &emp = mEmp;
                dBVector.push_back(&mEmp);
                dBVector[dBVector.size()-1]->printEmp();

            }

            else if (empInput == 2){
                cout << "Enter First Name: ";
                cin >> fNameInp;

                cout << "Enter Last Name: ";
                cin >> lNameInp;

                cout << "Enter Salary: ";
                cin >> salInp;

                cout << "Cpp Experience (Y/N): ";
                string cppInp;
                cin >> cppInp;

                cout << "Years of experience: ";
                float expInp;
                cin >> expInp;

                cout << "Engg Type (Chem, Mech, IT): ";
                string typInp;
                cin >> typInp;

                Engg eEmp(fNameInp, lNameInp, salInp, cppInp, expInp, typInp);
                Employee &emp = eEmp;
                dBVector.push_back(&eEmp);
                dBVector[dBVector.size() - 1]->printEmp();

            }

            else if (empInput == 3){
                cout << "Enter First Name: ";
                cin >> fNameInp;

                cout << "Enter Last Name: ";
                cin >> lNameInp;

                cout << "Enter Salary: ";
                cin >> salInp;

                cout << "School of PhD: ";
                string schoolInp;
                cin >> schoolInp;

                cout << "Topic of PhD: ";
                string topImp;
                cin >> topImp;

                Researcher rEmp(fNameInp, lNameInp, salInp, schoolInp, topImp);
                Employee &emp = rEmp;
                dBVector.push_back(&rEmp);
                dBVector[dBVector.size() - 1]->printEmp();

            }
            break;

        case 2: // Delete Emp
            for (int x = 0; x < dBVector.size(); x++){
                dBVector[x]->getLastName();
                cout << endl;
            }

            cout << "Input Last name of the employee to delete: " << endl;
            cin >> lNameSearch;
            for (i = 0; i < dBVector.size(); i++){
                if (dBVector[i]->getLastName() == lNameSearch){
                    dBVector.erase(dBVector.begin() + i);
                    cout << dBVector[i]->getFirstName() << "has been deleted from database";
                    deleted = true;
                    break;
                }
            }
            if (deleted == false && i == dBVector.size()){
                cout << "No Employee with Last Name - " << lNameSearch << " exists in Database." << endl;
            }
            else
                break;

        case 3: //save
            cout << "saving..." << endl;
            break;

        case 4: //exit
            exit = true;
            break;
        }
    }
}
Please Help!

Answer 1

您可能希望在向量中存储指针以避免切片，正如其他人提到的那样。然后每个员工都可以有自己的输入法，并提出正确的问题来初始化自己（main 不会实现它，而只会调用各自员工的虚拟输入函数）。输入和输出在概念上是对称的操作，暗示对称地实现它们，即在这种情况下都作为成员函数。

Answer 2

首先，如果你想使用多态性，你需要在你的向量中存储指针。由于 vector 是员工的唯一所有者，因此 std::vector<std::unique_ptr<Employee>> 之类的东西是合适的。

编辑：我看到您更新了矢量以使用指针。但是您正在存储指向本地堆栈分配对象的指针，例如 mEmp。这是行不通的，当 mEmp 变量在右大括号处超出范围时，对象将被删除，您将在向量中留下一个指向已删除对象的悬空指针。使用此悬挂指针是未定义的行为。您需要使用 new 在堆上分配 Manager。然后，当变量超出范围时，该对象将不会被删除，但您需要记住在完成后删除该对象。像 unique_ptr 这样的东西让这很容易。

关于您的问题：

尽量减少显式转换，尤其是向下转换。在将 Employee 存储在向量中时，它将从派生的 class 隐式向上转换为 Employee 但除此之外，不需要太多转换。
关于覆盖方法，您的想法大致正确，如果您在 Employee 指针上调用虚拟 printEmp 方法，它将调用派生的覆盖class。

如果您乐于让 Employee classes 负责用户输入，您可以简单地添加一个虚拟方法，使用来自用户的适当输入来初始化员工。但我很想将其与您的域对象分开。无论如何，您都需要一个关于员工类型的用户选择的 switch 语句，因此多态性在那里不会给您带来太多好处。

如果你真的想为员工创建使用多态性，我建议使用类似抽象工厂模式的东西。

无论如何，这是我的建议：

#include <vector>
#include <string>
#include <iostream>
#include <memory>

class Employee {
 public:
  Employee(std::string fName, std::string lName, int sal);
  virtual ~Employee();

  virtual void printEmp();

 protected:
  std::string m_fName;
  std::string m_lName;
  int m_sal;
};

class Manager : public Employee {
 public:
  Manager(std::string fName, std::string lName, int sal, int meets, int hols);
  void printEmp() override;

 protected:
  int m_meets;
  int m_hols;
};

Employee::Employee(std::string fName, std::string lName, int sal)
  : m_fName(fName), m_lName(lName), m_sal(sal) {
}

Employee::~Employee() {
}

void Employee::printEmp(){
  std::cout << "First Name: " << m_fName << "\n"
            << "Last Name: " << m_lName << "\n"
            << "Salary: " << m_sal << "\n";
}

Manager::Manager(std::string fName, std::string lName, int sal, int meets, int hols) 
  : Employee(fName, lName, sal), m_meets(meets), m_hols(hols){
}

void Manager::printEmp(){
  Employee::printEmp();
  std::cout << "Meets/Week: " << m_meets << "\n"
            << "Holidays/Year: " << m_hols << "\n";
}

std::unique_ptr<Manager> createManager() {
  std::cout << "Enter First Name: ";
  std::string fNameInp;
  std::cin >> fNameInp;

  std::cout << "Enter Last Name: ";
  std::string lNameInp;
  std::cin >> lNameInp;

  std::cout << "Enter Salary: ";
  int salInp;
  std::cin >> salInp;

  std::cout << "Number of meetings/week: ";
  int meetsInp;
  std::cin >> meetsInp;

  std::cout << "Number of holidays/year: ";
  int holsInp;
  std::cin >> holsInp;
  std::cout << "\n";

  return std::make_unique<Manager>(fNameInp, lNameInp, salInp, meetsInp, holsInp);
}

std::unique_ptr<Employee> createEmployee() {
  int input;  
  std::cout << "1) Add a Manager, 2) Add an Engg, 3) Add a Researcher\n";
  std::cin >> input;    
  switch (input){
    case 1:
      return createManager();   
    default:
      return nullptr;
  }
}

int main() {
  std::vector<std::unique_ptr<Employee>> dBVector; 

  std::cout << "Welcome to Employee Database, Enter an option to continue...\n";
  std::cout << "1) Add an Employee"
            << ", 2) Delete an Employee"
            << ", 3) Save Database"
            << ", 4) Exit\n";
  int input;
  std::cin >> input;

  switch (input){
    case 1:
      dBVector.push_back(createEmployee());
      break;
    default:
      break; // Do nothing
  }

  dBVector.at(0)->printEmp();    
}

Live demo

创建基础 class 对象的向量并将派生的 class 对象存储在其中

Create a vector of base class objects and store derived class objects within

c++

polymorphism

inheritance