Что-то с конструктором

#include <iostream>
using namespace std;
 
class Tail
{
public:
    Tail()
    {
        addTail();
    }
    void addTail();
    Tail* next;
};
 
Tail* first = 0;
 
void Tail::addTail()
{
    if (first == 0)
    {
        first = this;
    }
    else
    {
        Tail* temp;
        for (temp = first; temp->next; temp = temp->next){  }
        temp->next = this;
    }
    next = 0;
}
 
class Account : public Tail
{
public:
    Account::Account(unsigned accNo, double balance = 0.0)
    {
        numberAcc = accNo;
        balance = 0.0;
    }
    void deposit(double transaction)
    {
        balance += transaction;
    }
    virtual void operation(double transaction) = 0;
    virtual void withdraw(double transaction) = 0;
    void display()
    {
        cout << balance << endl;
    }
protected:
    double balance;
    unsigned numberAcc;
};
 
class Checking : public Account
{
public:
    Checking::Checking(unsigned accNo, double balance = 0.0) : Account(accNo, balance = 0.0){
        balance = 0.0;  
    }
    virtual void operation(double transaction);
    virtual void withdraw(double transaction);
};
 
class Saving : public Account
{
    public:
    Saving::Saving(unsigned accNo, double balance = 0.0) : Account(accNo, balance = 0.0){balance = 0.0;}
    virtual void withdraw(double transaction);
    virtual void operation(double transaction);
};
 
unsigned total = 0;
 
void Checking::operation(double transaction)
{
    if (balance < 500)
        balance -= 0.2;
    balance += transaction;
}
 
void Checking::withdraw(double transaction)
{
    if (balance < 500)
        balance -= 0.2;
    balance -= transaction;
}
 
void Saving::operation(double transaction)
{
    balance += transaction;
}
 
void Saving::withdraw(double transaction)
{
    total++;
    if (total >= 1)
        balance -= 5.0;
    balance += transaction;
}
 
void process(Account* el)
{
    double transaction;
    do
    {
        cout << "Please enter transaction: ";
        cin >> transaction;
        if (transaction > 0)
            el->operation(transaction);
        else
            el->withdraw(-transaction);
    }while(transaction != 0);
}
 
int getNumber()
{
    unsigned number;
    cout << "Enter number of account: ";
    cin >> number;
    return number;
}
 
int main()
{
    Account* el;
    while(1)
    {
        char type;
        cout << "S = Saving, C = Checking, X = exit" << endl;
        cin >> type;
        if (type == 'X' || type == 'x')
            break;
        switch(type)
        {
        case 'S':
        case 's':
            el = new Saving(getNumber());
            break;
        case 'C':
        case 'c':
            el = new Checking(getNumber());
            break;
        default:
            cout << "I can't understund your symbol. Try again!" << endl;
        }
        el->next = 0;
        process(el);
        el->display(); // <--- вот тут выводим баланс только что введенного счета, но почему-то всегда выводится число -6.27....
    }
    system("pause");
    return 0;
}

/////////////////
Checking::Checking(unsigned theAccNo, double theBalance = 0.0) : Account(theAccNo, theBalance = 0.0){
....
 
/////////////////////////
 
//в мэйне:
Checking c(2., 2.);