误解多态C++

Question

我有四个班,银行,帐户,保存和检查.保存和检查都是从Account公开继承的.我在Account中有两个虚拟空函数; 存款和取款.我只发布了存款存款功能的代码,因为其余代码的问题是重复的.

我的Bank类中有一个函数,它将一个帐户添加到account类型的向量中.每当我为存储对象调用存款函数时,它使用Account的存款功能而不是保存(使用调试器找到).

起初我没有使用指针,但我经历了这个线程:多态性误解/重新定义的虚函数无法正常工作并学习使用指针来实现虚函数.

问题:什么导致我的代码使用Account.cpp中的默认虚拟方法而不是Saving.cpp中的预期"多态"方法？我该如何解决？

Account.cpp

#pragma once
#include <string>
using std::string;
enum account_type { saving, checking };

class Account
{
public:
    Account();
    Account(int, account_type, double);
    ~Account();

    virtual void deposit(double&) {};
    virtual void withdraw(double&) {};
protected:
    int account_number;
    account_type type;
    double balance;

    int generateAccountNumber();
    double initializeBalance();
};

Saving.cpp

class Saving : public Account
{
public:
    Saving();
    Saving(int, account_type, double);
    ~Saving();

    void deposit(double&) //deposits some amount into a saving account
    {
        if (amount < 0)
            throw "Cannot withdraw an amount less than $0.00";
        else if (amount > balance)
            throw "Cannot withdraw an amount greater than the current balance";
        else
            balance -= amount;
    }
 private:
    const double interest_rate = 0.01;
};

Bank.cpp

class Bank
{
private:
    vector <Account*> bank_accounts;
    //ORIGINAL BEFORE FIX: vector <Account> bank_accounts;
public:
    void Bank::addAccount(Account a) //adds some account to a vector
    {
        bank_accounts.push_back(a);
    }

    Account * findAccount(int acc_num) //finds the account by it's account number
    { 
        int found = -1;
        for (int y = 1; y <= (int)bank_accounts.size(); y++) {
        if (acc_num == bank_accounts[y - 1].getAccountNumber())
            found = y - 1;
        }

         if (found == -1) {
             throw "\nAccount not found";
         }
        else
        {
            if (bank_accounts[found].getAccountType() == 0)
            {
                Account * saving = &bank_accounts[found];
                return saving;
            }
        else if (bank_accounts[found].getAccountType() == 1)
        {
                Account * checking = &bank_accounts[found];
                return checking;
        }
     }
  }
}

int main()
{
    Bank Swiss;
    Saving s(num, type, bal); // some user values for these variables
    Account * tempAccount1 = &s;
    Swiss.addAccount(*tempAccount1);
    Swiss.findAccount(num)->deposit(amt);
}

Answer 1

我没有看到执行此操作的代码,但问题是说它Bank有一个帐户向量,即std::vector<account>.如果是这种情况,问题是派生类对象account在被推入向量时会被切成对象,并且它们会失去派生的对象.代码需要使用指针或引用account.通常,这是使用std::vector<account*>或std::vector<std::unique_ptr<account>>分配的派生对象完成的new.代码还应该传递account由指针或引用派生的类型的对象,而不是通过值(例如,void Bank::addAccount(Account a)因为它将对它所调用的参数进行切片而不起作用).