Question

Question says it all, really. I’m not sure what the problem is. I’m fairly new to classes; my practical experience with them being close to nill, but I have read a fair amount about them.

I have created a class ECard with the following constructor

ECard::ECard( int bankNum, int PIN )
{
    m_BankNum     = new int;
    m_PIN         = new int;
    m_Barred      = new bool;
    m_Amount      = new double;

    *m_BankNum     = bankNum;
    *m_PIN         = PIN;
    *m_Barred      = false;
    *m_Amount      = 100.0;
}

and I initialize with EC card( 12345, 54321 )

I also have a member function display(), which simply prints out all the member variables BankNum, PIN, Barred, and Amount.

When I call this function card.display() in my main function, the output is perfectly what I expected.

However, when it enters my loop:

/* Fine values! */
card.display();
while( true )
{
    /* Introductory screen giving user options to choose from */
    mainScreen( card );
    /* Make a choice... */
    choice = readInput();
    /* Garbage! */
    card.display();
    /* Pass it to the switch, watch out for invalid input! doChoice is a bool */
    if( !doChoice( choice, card ) )
    {
        cout << "Bad input- repeat!" << endl;
    }
    /* TODO: Option to terminate loop. */
}

and I try to print the variables in my doChoice() function, I get garbage. All my variables have messed up settings. My Banknumber is something, PIN is a really large negative number( not the MIN ), Barred is suddenly set to true, and there is 0 money in my account even though Barred and Amount were never explicitely set by me anywhere outside the constructor.

/* Outsourced switch that handles the user input from MAIN */
bool doChoice( int choice, ECard card )
{
    int inputPIN;
    int inputAmount;

    /* Garbage! */
    card.display();

    switch( choice )
    {
    case 1:
    /* Case 1: Charge card with money. Needs PIN and amount */
        cout << "PIN Eingeben:  ";
        inputPIN    = readInput();
        cout << "\nBetrag Eingeben:  ";
        inputAmount = readInput();

        karte.aufladen( inputAmount, inputPIN );

        return true;

I’m sorry if some of the member functions and the outputs are still in German. It shouldn’t be important anyway( I like to write all my homework in English anyway since I’ll probably be programming for international companies, but my teacher is really picky and lowers my grade if I program outside of his expectations )

The variables are already messed up before it even enters the switch, so that part should just be rhetoric. I can see a problem only with the passing of the card object, though I don’t know why that would be a problem. I don’t know how to fix it, do I just send each member instead? Create a pointer to the object and send that? I mean, I passed the card object around before and the other function didn’t give me garbage. Only this one.

On a final note, if there suddenly is an error in syntax, then that is because I quickly translated all my functions and members into English, and I may have missed a capitalization somewhere or whatnot. Please disregard those, the syntax is correct and the program runs, it just displays garbage values.

Answer 1

Don’t dynamically allocate variables unless necessary

In the Java language, instances of variables are defined using the new operator. This is not necessary in C++.

Change your class declaration to:

struct ECard
{
    ECard(unsigned int bankNum,
          unsigned int newPIN)
    : m_bankNum(bankNum),
      m_pin(newPIN),
      m_barred(false),
      m_amount(100.0)
  {
    ;
  }

  unsigned int m_bankNum; // Can bank numbers be negative???
  unsigned int m_pin;
  bool         m_barred;
  double       m_amount;
};

Pointers

If you really need to dynamically allocate memory for the members, use smart pointers. Smart pointers will automagically deallocate memory memory when needed. I suggest using the Boost library ones.

If you can’t use smart pointers, remember to deallocate the memory in the destructor:

ECard::
~ECard()
{
  delete m_p_bankNum;
  delete m_p_pin;
  delete m_p_barred;
  delete m_p_amount;
}

If you don’t use pointers in your class or structure, you avoid the hassles of dynamic memory allocation and deallocation, especially for copying and destroying (who owns the memory?).