"cannot declare abstract type variable because of virtual function" 错误，尽管虚函数是 defined/implemented

Question

我正在使用继承编写代码，基 class 称为 BankAccount，子 class 称为 MoneyMarket Account，我的代码

hw7main.cpp: In function ‘int main()’:
hw7main.cpp:9: error: cannot declare variable ‘account1’ to be of abstract type ‘MoneyMarketAccount’
hw7.h:36: note:   because the following virtual functions are pure within ‘MoneyMarketAccount’:
hw7.h:41: note:     virtual bool MoneyMarketAccount::withdraw(double)

根据其他类似问题的答案，我尝试重写虚函数，withdraw，但仍然出现同样的错误。

这是我的基础class界面（文件名：hw7base.h）：

#ifndef HW7BASE_H
#define HW7BASE_H
#include <iostream>
#include <string>
using namespace std;

class BankAccount
{
    public:
    //constructors
    BankAccount();

    //member functions
    bool deposit(double money);
    virtual bool withdraw (double money)=0;

    //accessor functions
    void getName(BankAccount* account);
    void getBalance(BankAccount* account);

    //transfer function
    //virtual void transfer (BankAccount* deposit, BankAccount* withdrawal, double money);
    //private:
    string name;
    double balance;
};


#endif

这是我的子class界面（文件名：hw7derived1.h）：

#ifndef HW7DERIVED1_H
#define HW7DERIVED1_H
#include <iostream>
#include "hw7base.h"

using namespace std;

class MoneyMarketAccount: public BankAccount
{
    public:
        //constructor
        MoneyMarketAccount();
        //override function
        virtual bool withdraw(double money)=0;
        int number_of_withdrawals;

};

#endif

这是我的子class实现（文件名：hw7derived1.cpp）：

#include "hw7derived1.h"
using namespace std;

//consturctor
MoneyMarketAccount::MoneyMarketAccount()
{
    number_of_withdrawals = 0;
}
bool MoneyMarketAccount::withdraw(double money)
{
    if (money<=0)
    {
        cout<<"Failed.\n";
        return false;
    }
    else
    {
        balance-=money;
            if(number_of_withdrawals>=2)
            {
                balance-=1.5;
                if (balance<0)
                {
                    balance=balance+1.5+money;
                    cout<<"Failed.\n";
                    return false;
                }
                else
                {
                    number_of_withdrawals++;
                    cout<<"Success.\n";
                    return true;
                }
            }
            else
            {       
                if (balance<0)
                {
                    balance+=money;
                    cout<<"Failed.\n";
                    return false;
                }
                else
                {
                    number_of_withdrawals++;
                    cout<<"Success.\n";
                    return true;
                }
            }
    }
}

任何 help/insight 不胜感激，谢谢！

Answer 1

这个：

virtual bool withdraw(double money)=0;

(In MoneyMarketAccount - 派生的 class) 将函数声明为纯函数（就像它在基础 class 中是纯函数一样）。即使你实现了它（所以它 can 被调用 - 是的，纯虚函数 can 有实现并且它有时确实有意义），这仍然使 MoneyMarketAccount 抽象 - 你不能实例化抽象 class.

您可能想将函数声明为

bool withdraw(double money) override;

在 MoneyMarketAccount 中。这将使 class 具体化，因为不再有任何纯 (=0) 函数，并且如果基础 class 签名发生变化，编译器也会在将来帮助您，因此您不再从它覆盖纯虚拟 withdraw（override 位 - 这使得派生的 class 中的冗余 virtual 额外冗余）。

"cannot declare abstract type variable because of virtual function" 错误，尽管虚函数是 defined/implemented

"cannot declare abstract type variable because of virtual function" error despite the virtual function being defined/implemented

c++

virtual-functions