無法創建抽象類的實例

Question

嘗試創建此類的實例時，此代碼存在問題，似乎是一個錯誤

not allowed to use the abstracted class "SavingAccount"

我不知道該怎么辦。 我按照edx Microsoft Intermediate C ++給出的步驟進行操作。

銀行賬戶

#pragma once
#include <string>
class BankAccount
{
protected: 
    double balance;
public:

BankAccount(double initialBanlance);
virtual ~BankAccount();

double getBalance() const;

virtual void deposit(double amount);
virtual void withdraw(double amount);

virtual std::string getTermAndConditions() = 0;
virtual double getGuaranteeLimit() = 0;};

銀行賬戶

#include "BankAccount.h"

BankAccount::BankAccount(double initialBanlance)
:balance(initialBanlance)
{}

BankAccount::~BankAccount()
{}

double BankAccount::getBalance() const
{return balance;}

void BankAccount::deposit(double amount)
{
balance += amount;
}

void BankAccount::withdraw(double amount)
{balance -= amount;}

---

Freezable.h

#pragma once
//Pure virtual class ,representing the "freeable" capability
class Freezable {
public:
virtual void freeze()=0;
virtual void unfreeze()=0;
};

---

可記錄的

#pragma once
#include <string>
//Pure virtual class, representing the "loggable"'
class loggable{
public:
virtual void log(const std::string& message)const = 0;};

---

SavingAccount.h

#pragma once
#include "BankAccount.h"
#include "Freezable.h"
#include "Loggable.h"

#include <list>
class SavingAccount :public BankAccount, public Freezable, public loggable
{
private:
    double interestRate;
    bool frozen;
public:
    SavingAccount(double initialBalance, double interestRate = 0.0);
    virtual ~SavingAccount();

    void earnInterest();

    virtual void deposit(double amount);
    virtual void withdraw(double amount);

    //implement  pure virtual function from BankAccount class.
    virtual std::string getTermAndConditions();
    virtual double getGuarranteeLimit();

    //Implement pure virtual from Freezable
    virtual void freeze();
    virtual void unfreeze();

    //Implement pure virtual from Loggable class
    virtual void log(const std::string & message)const;
};

SavingAccount.cpp

 #include "SavingAccount.h"
 #include <iostream>

 SavingAccount::SavingAccount(double initialBalance, double interestRate)
    :BankAccount(initialBalance), interestRate(interestRate), frozen(false)
{}    
SavingAccount::~SavingAccount() {}    
void SavingAccount::earnInterest()
{
    if (!frozen) {
        double interest = balance * (interestRate / 100);
        deposit(interest);
    }
}    
void SavingAccount::deposit(double amount) {
    if (!frozen) {
        BankAccount::deposit(amount);
        log("Deposit:" + std::to_string(amount));
    }
}    
void SavingAccount::withdraw(double amount) {
    if (!frozen && amount <= balance) {
        BankAccount::withdraw(amount);
        log("withdrwal:" + std::to_string(amount));
    }
}    
std::string SavingAccount::getTermAndConditions() {
    return "This is a savings account, You cannot go overdrawn.You  earn interest.";
}    
double SavingAccount::getGuarranteeLimit() {    return 1000000; }

void SavingAccount::freeze() {  frozen = true; }

void SavingAccount::unfreeze() {    frozen = false; }

void SavingAccount::log(const std::string & message) const 
{   std::cout << message << std::endl; }

Answer 1

你有錯字 基類BankAccount是一個純抽象類 ，它具有一個虛擬成員函數，稱為

virtual double getGuaranteeLimit() = 0;
                    ^^

並在您的SavingAccount類（派生的）中實現了

virtual double getGuarranteeLimit();
                     ^^

這與基類不同 。 因此，您永遠不會在派生類中覆蓋該函數。 這就是為什么您需要使用重寫說明符進行練習的原因，如果在基類中找不到匹配的函數，這將產生編譯器錯誤。

例如，參見使用Clag 7.0進行編譯顯然會出現編譯器錯誤：

prog.cc:76:17: error: 'getGuarranteeLimit' marked 'override' but does not override any member functions
        virtual double getGuarranteeLimit()override;