Expand and add a new object to a array inside a function

Question

I'm trying to expand and add a new object to a array inside a function and have that array be effected outside the function as well (the arrays pointer is sent as a parameter).

void addMedia(Media* medias[], int &nrOfMedias, string title, int publYear, string author, int nrOfPages)
{
    Media** tempArray = new Media*[nrOfMedias +1];
    for(int i = 0; i < nrOfMedias; i++)
    {
        tempArray[i] = medias[i];
    }
    delete [] medias;
    medias = tempArray;
    delete [] tempArray;
    medias[nrOfMedias] = new Book(title, publYear, author, nrOfPages);
    nrOfMedias++;
}

This code works great inside the function but when I get outside it the array is still empty. As i understand this it's because the pointer is changed inside the function but how can i expand the array without having it change the pointer?

(I can not change the return data type or the parameters, assignment requirements.)

Use vectors, and pass them as references. Much, much simpler/safer. (BTW, the second delete in there looks really fishy.) — Mat
– Mat, Commented May 14, 2012 at 17:42
I can not change the return data type or the parameters Then it's impossible to do what you want. — cnicutar
– cnicutar, Commented May 14, 2012 at 17:43
Yeah, I had to delete my answer that said use vectors because I saw this requirement. — boiler96
– boiler96, Commented May 14, 2012 at 17:46
How is your array defined and allocated outside the function? — jrok
– jrok, Commented May 14, 2012 at 17:51

Greg · Accepted Answer · 2012-05-14 18:00:50Z

2

Do change medias = tempArray; to *medias = tempArray;, make it compile, polish your memory management (consider, what really should be freed, what not).

answered May 14, 2012 at 18:00

Greg

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4 Comments

Dragon Creature Over a year ago

did you mean? *medias = *tempArray; Got it to work with this code and removed delete [] medias; thx.

Greg Over a year ago

No, I mean *medias = tempArray, exactly as Rollie answer.

Dragon Creature Over a year ago

that give me an error. IntelliSense: a value of type "Media **" cannot be assigned to an entity of type "Media *"

Greg Over a year ago

That's what I meant by "make it compile" :-). See Rollie answer.

Rollie · Accepted Answer · 2012-05-14 18:45:53Z

2

Don't view medias as an array of pointers, view it as a pointer to an array. Working example (slightly simplified):

class Media
{   
public:
    Media () { m_strTitle = "unknown";}
    string m_strTitle;
};

class Book : public Media
{
public:
    Book(string strTitle) { m_strTitle = strTitle; }
};

void addMedia(Media* medias[], int &nrOfMedias)
{
    Media * tempArray = new Media[nrOfMedias +1];
    for(int i = 0; i < nrOfMedias; i++)
    {
        tempArray[i] = (*medias)[i];
    }
    delete [] *medias;
    (*medias) = tempArray;

    (*medias)[nrOfMedias] = Book("newTitle");
    nrOfMedias++;
}

int main()
{
    int numMedia = 10;
    Media * myArray = new Media[numMedia];
    addMedia(&myArray, numMedia);
    for (int i = 0; i < numMedia; i++)
    {
        cout << i << ") " << myArray[i].m_strTitle << endl;
    }

    return 0;
}

edited May 14, 2012 at 18:45

answered May 14, 2012 at 18:12

Rollie

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2 Comments

Dragon Creature Over a year ago

Ok thx the code seems to work fine for one problem. I should probably have mention this earlier but Media is a abstract class so it doesn't take well to first line of code in addMedia.

Rollie Over a year ago

If that is the case, and you can't change the function signature, I believe it is impossible to alter the passed in medias array.

Terenty Rezman · Accepted Answer · 2012-05-14 18:34:35Z

0

You don't need delete [] tempArray; because tempArray actually points to the same memory block as medias does after medias = tempArray;

Your function will work well whithout that line but I assume that you know what you pass with Media* medias[]

answered May 14, 2012 at 18:34

Terenty Rezman

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