C ++继承的类未正确初始化

Question

I have a class (SavingAccount) inherited from another class (Account). Somehow, it does not initialize properly.

========================================================================

class Account
{
private:
    double balance;

public:
    // constructor
    Account()
    {
        balance = 0;
    }

    Account(double b)
    {
        if (b >= 0)
            balance = b;
        else
        {
            balance = 0;
            cout << "Balance cannot be negative." << endl;
        }
    }
    // destructor
    ~Account()
    {
        // nothing
    }

    // required functions
    bool credit(double a)
    {
        if (a > 0)
        {
            balance += a;
            return true;
        }
        else
        {
            cout << "Credit amount must be greater than 0." << endl;
            return false;
        }
    }

    bool debit(double a)
    {
        if (a > 0 && a <= balance)
        {
            balance -= a;
            return true;
        }
        else if (a > balance)
        {
            cout << "Debit amount exceeded account balance." << endl;
            return false;
        }
    }

    double getBalance()
    {
        return balance;
    }

    void setBalance(double b)
    {
        balance = b;
    }   
};

========================================================================

class SavingAccount : public Account
{
private:
    double interestRate;    // percentage

public:
    // constructor
    SavingAccount()
    {
        Account::Account();
        interestRate = 0;
    }

    SavingAccount(double b, double r)
    {
        if (b >= 0 && r >= 0)
        {
            Account::Account(b);
            interestRate = r;
        }
        else
        {
            Account::Account();
            interestRate = 0;
            cout << "Balance or rate cannot be negative." << endl;
        }
    }
    // destructor
    ~SavingAccount()
    {
        // nothing
    }

    // required functions
    double calculateInterest()
    {
        return Account::getBalance() * interestRate / 100;
    }
};

========================================================================

I intialize as follow:

    SavingAccount acctSaving(2000, 5);

acctSaving shows its balance as 2000 when I jump inside it. However, when it gets outside, the balance is back to 0.

If I change this line:

Account::Account(b);

into:

setBalance(b);

It returns the object properly with the balance value of 2000. What goes wrong? How is the first method not right?

Answer 1

Account::Account(b) doesn't do what you seem to think it does. It creates an unnamed temporary variable of type Account , initialized with b , and then immediately destroyed.

What you are likely looking for is something like this:

SavingAccount(double b, double r)
  : Account(b >= 0 && r >= 0 ? b : 0) {
    /* rest of code here */
}

See also: member initializer list

Answer 2

You should not call parent constructors directly. But you can specify which parent constructor should be used when a child is being constructed.

When you call a parent constructor directly like a function call, you create a temporary object.

// constructor
SavingAccount() // Default parent constructor is called here implicitly
{
    interestRate = 0;
}

SavingAccount(double b, double r): Account(b) // Specify here parent constructor if default one is useless here
{
    if (b >= 0 && r >= 0)
    {
        interestRate = r;
    }
    else
    {
        interestRate = 0;
        cout << "Balance or rate cannot be negative." << endl;
    }
}