C++抽象类对象创建及员工管理类实现问题求助

Hey there! Let's tackle your two questions one by one, and fix that frustrating "cannot create abstract class object" error you're running into with your code.

Short answer: No, you can't directly create an object of an abstract class. This is intentional—abstract classes are designed to define a common interface that derived classes must implement, not to be instantiated on their own. The presence of at least one pure virtual function (like your virtual void print()=0;) marks the class as abstract.

Instead, you work with pointers or references to the abstract class type. These pointers/references can point to objects of concrete derived classes (classes that implement all pure virtual functions from the abstract base). This is how polymorphism works in C++—it lets you treat all employee types the same way while still calling their specific behaviors.

Let's break down why you're getting that error first, then walk through a corrected version of your code:

Why your current code fails: #

• Your PermanentEmployee and ContractualEmployee classes don't implement the pure virtual print() function from Employee. That makes them abstract classes too—so you can't create objects of them either!
• You tried to declare PermanentEmployee PE; without a default constructor (your PermanentEmployee only has a parameterized constructor).
• PermanentEmployee uses protected inheritance, which breaks the "is-a" relationship with Employee—you won't be able to use it where an Employee* is expected, which defeats the purpose of polymorphism for your company's employee list.
• Your Employee constructor is declared but not implemented, which would cause a linker error later.
• The Company class's list is a raw pointer, which isn't flexible enough to store an arbitrary number of employees.

Corrected Code #

#include<iostream>
#include<string>
#include<vector> // For dynamic, resizable employee storage
using namespace std;

struct dob { int date, month, year; }; // Fixed typo: "day" -> "month" (likely a mistake)

class Employee {
public:
    string Name;
    int ID;
    string Department;
    double MonthlyGrossSalary;
    double MonthlyNetSalary;

    // Implement the base class constructor
    Employee(string name, int id, string dep, double mgs, double mns)
        : Name(name), ID(id), Department(dep), MonthlyGrossSalary(mgs), MonthlyNetSalary(mns) {}

    // Empty implementations for now (you can expand these later)
    virtual void CalculateAnnualIncomeTax() {}
    virtual void CalculateMonthlyIncomeTax() {}
    virtual void print() = 0; // Pure virtual function - MUST be implemented by derived classes
};

// Use public inheritance to maintain "is-a Employee" relationship
class PermanentEmployee : public Employee {
public:
    double ProvidentFund;
    double AnnualMedicalInsuranceLimit;

    PermanentEmployee(double pf, double amil, string name, int id, string dep, double mgs, double mns)
        : Employee(name, id, dep, mgs, mns), ProvidentFund(pf), AnnualMedicalInsuranceLimit(amil) {}

    // Implement the pure virtual print function
    void print() override {
        cout << "Permanent Employee Details:\n";
        cout << "Name: " << Name << "\nID: " << ID << "\nDepartment: " << Department << "\n";
        cout << "Gross Salary: " << MonthlyGrossSalary << "\nNet Salary: " << MonthlyNetSalary << "\n";
        cout << "Provident Fund: " << ProvidentFund << "\nMedical Limit: " << AnnualMedicalInsuranceLimit << "\n";
    }
};

class ContractualEmployee : public Employee {
public:
    dob ContractStartDate;
    dob ContractEndDate;

    ContractualEmployee(dob csd, dob ced, string name, int id, string dep, double mgs, double mns)
        : Employee(name, id, dep, mgs, mns), ContractStartDate(csd), ContractEndDate(ced) {}

    // Implement the pure virtual print function
    void print() override {
        cout << "Contractual Employee Details:\n";
        cout << "Name: " << Name << "\nID: " << ID << "\nDepartment: " << Department << "\n";
        cout << "Gross Salary: " << MonthlyGrossSalary << "\nNet Salary: " << MonthlyNetSalary << "\n";
        cout << "Contract Start: " << ContractStartDate.date << "/" << ContractStartDate.month << "/" << ContractStartDate.year << "\n";
        cout << "Contract End: " << ContractEndDate.date << "/" << ContractEndDate.month << "/" << ContractEndDate.year << "\n";
    }
};

class Company {
private:
    vector<Employee*> employees; // Dynamic list to store any number of employees
public:
    // Add a new employee to the company
    void AddEmployee(Employee* emp) {
        employees.push_back(emp);
    }

    // Print details of all employees in the company
    void PrintAllEmployees() {
        for (auto emp : employees) {
            emp->print();
            cout << "-------------------------\n";
        }
    }

    // Clean up dynamically allocated memory to avoid leaks
    ~Company() {
        for (auto emp : employees) {
            delete emp;
        }
    }
};

void datainput(string&, int&, string&, double&, double&);
void getContractDates(dob&, dob&);

int main() {
    char option;
    string name;
    string dep;
    int id;
    double mgs;
    double mns;
    Company myCompany;

    do {
        cout << "\nSelect Type of Employee\n";
        cout << "Permanent Employee [P][p]\n";
        cout << "Contractual Employee [C][c]\n";
        cout << "Quit & Print All [Q][q]\n";
        cout << "Enter your choice: ";
        cin >> option;

        switch(option) {
            case 'P':
            case 'p': {
                datainput(name, id, dep, mgs, mns);
                double pf, amil;
                cout << "Enter Provident Fund: ";
                cin >> pf;
                cout << "Enter Annual Medical Insurance Limit: ";
                cin >> amil;
                // Create employee dynamically and add to company
                myCompany.AddEmployee(new PermanentEmployee(pf, amil, name, id, dep, mgs, mns));
                cout << "Permanent employee added!\n";
                break;
            }
            case 'C':
            case 'c': {
                datainput(name, id, dep, mgs, mns);
                dob start, end;
                getContractDates(start, end);
                // Create employee dynamically and add to company
                myCompany.AddEmployee(new ContractualEmployee(start, end, name, id, dep, mgs, mns));
                cout << "Contractual employee added!\n";
                break;
            }
            case 'Q':
            case 'q':
                cout << "\n--- All Company Employees ---\n";
                myCompany.PrintAllEmployees();
                break;
            default:
                cout << "Invalid option! Try again.\n";
        }
    } while (option != 'Q' && option != 'q');

    return 0;
}

void datainput(string& n, int& no, string& d, double& mg, double& mn) {
    cout << "\nEnter Employee Data\n";
    cout << "Name: ";
    cin >> n;
    cout << "ID: ";
    cin >> no;
    cout << "Department: ";
    cin >> d;
    cout << "Monthly Gross Salary: ";
    cin >> mg;
    cout << "Monthly Net Salary: ";
    cin >> mn;
}

void getContractDates(dob& start, dob& end) {
    cout << "\nEnter Contract Start Date (date month year): ";
    cin >> start.date >> start.month >> start.year;
    cout << "Enter Contract End Date (date month year): ";
    cin >> end.date >> end.month >> end.year;
}

Key changes made: #

1. Implemented pure virtual print(): Both derived classes now have their own print() implementation, making them concrete (non-abstract) classes that can be instantiated.
2. Public inheritance: PermanentEmployee now uses public inheritance, so it's properly recognized as an Employee type for polymorphism.
3. Dynamic storage with std::vector: The Company class uses vector<Employee*> to store any number of employees, which is flexible and safe.
4. Dynamic object creation: Employees are created with new (and cleaned up in the Company destructor) so they can be stored as Employee* pointers.
5. Added loop for multiple employees: The main function lets you add as many employees as you want until you quit, then prints all of them.

内容的提问来源于stack exchange，提问作者Rimsha Nasir