Dynamic array of objects

Question

I'm aware that I could use something called std::vector, but I'm afraid it's not possible because of the course restrictions.

I need to make a dynamic extensible array of objects. The array should grow and grow when new objects need to be stored.

Here is the class that the array belongs to:

class TransactionList
{
    private:
        Transaction *trans;
        int amountTransactions;
        Transaction newTrans;

    public:
        TransactionList();
        ~TransactionList();
        void read( istream &is );
        void write( ostream &os );
        void add( Transaction & newTrans ); 
        double totalExpenses();
        double hasPaid( string namnet );
        double isDebted( string namnet );
        //PersonList FixPersons();
 };

The method "void add ( Transaction & newTrans )" is the one I need. And yes, I seriously have to do it pointer-style.

So far this method is totally incomplete and just not even close to functional. I've tried several different ways, but end up with a runtime error or just bollocks result.

void TransactionList::add(Transaction & newTrans)
{
    Transaction* tempArrPtr;

    amountTransactions++;
    trans = new Transaction[amountTransactions]
    trans[amountTransactions - 1] = newTrans;
}

What I want the method to do is to build an array of of Transaction-objects and grow in size while it gets more objects.

I hope I've written about my problem clearly and wish someone could give me a good answer. I tried Googling, but I'm still stuck - otherwise I wouldn't have bothered asking :p

Also if someone could give some pointers about copy constructors, I'd be very thankful. In our course material they pretty much didn't even show what a copy constructor should look like. Just like "You need copy constuctors for deep copying, good luck!"

Answer 1

You should add a maxTransactions variable, which would indicate the allocated length of your trans* array, and initialize both ammountTransactions and maxTransactions with 0.

Your array would automatically double its size when we reach the limits of trans

void TransactionList::add(Transaction & newTrans)
{
    if(amountTransactions == maxTransactions){ //we've reached the capacity of trans
        //allocate a new array
        Transaction* nuTrans = new Transaction[maxTransactions*2+1];
        //copy the old values of trans into nuTrans
        memcpy(nuTrans, trans, sizeof(Transaction)*maxTransactions);
        //deallocate the old trans array
        delete []trans;
        //set trans to point at your newly allocated array
        trans = nuTrans;
        //update maxTransactions
        maxTransactions = maxTransactions*2+1;
    }

    trans[amountTransactions] = newTrans;
    amountTransactions++;
}

PS. I wrote it directly here, I didn't check it if it compiles as a whole or didn't debug the code. But I present it as an idea you could follow

Edit: Working example @ http://ideone.com/uz1mE

Answer 2

When you add an object and the array is too small you need to create a new one with the correct or larger size, copy the data, delete the old one and replace it with your new one.

Copy constructors are just like ordinary constructors only that they take an object of the same type. Do remember take care of your pointers properly when doing this.

 TransactionList(const TransactionList & o);

Answer 3

Now I finally managed to solve this puzzle. Turns out I wasn't paying enough attention to the fact that my Transaction-objects themselves held a dynamic array, so I finally came up with the idea of making an assign function to copy the objects. Thought I'd share my solution just in case any one has to tackle the same problem with the same limited set of tools.

This is how it ended up looking like:

   void TransactionList::add(Transaction & newTrans)

   {
    amountTransactions++;

    cout << "Adding a transaction-object to the array. amountTransactions = " << amountTransactions << endl;
    //Allocate a new array
    Transaction* tempTrans = new Transaction[amountTransactions];
    //Copy the objects with the assign-function
    for (int i = 0; i < amountTransactions - 1; i++)
            tempTrans[i].assign(trans[i]);

    //Delete the old one
    delete[] trans;

    //Set trans to point at the new one
    trans = tempTrans;

    //Add the newcomer object
    trans[amountTransactions - 1].assign(newTrans);
}

And the assign-function looks as follows:

void Transaction::assign(const Transaction & t)
{
    date = t.date;
    type = t.type;
    name = t.name;
    amount = t.amount;
    amountFriends = t.amountFriends;

    cout << "Hello assign " << amountFriends << endl;

    delete [] friends;
    if (amountFriends > 0)
    {
        friends = new string[amountFriends];

        for (int i = 0; i < amountFriends; i++)
            friends[i] = t.friends[i];
    }

    else
        friends = NULL;
}

I based my final solution on matyas' answer, so I owe you one buddy! :) Thanks also to Alexandre C. for good read!

I'm not counting out the possibility there might be some error in the code, but at least it compiles, runs and produces correct result. Feel free to point out if you find something that's not right.