Constructing an array of class attribute in a different class C++

Question

I have to do a project where one class Row has an array of integers int* OneArray and then another class Array has an array of the first class Row* TwoDArray. In essence the class has a 2D array on integers, and I can easily do the construction of the 2D array when it is in one class. However, now I am completely stumped.

The construction of the Row is simple enough:

//set length of array
numOfRows = intRows;
//Create space for array
OneArray = new int[intRows];
//populate array with random numbers
for(int i=0; i<intRows; i++)
{
    OneArray[i] = GenerateRandom(9,0);
}

This is where I am stuck (Construction of Array):

//Set Number of Cols
NumOfCol = intCols;
//create length for each row
int intLength = 4;
for(int i=0; i<NumOfCol; i++)
{
    //create space and call row constructor with length
    TwoDArray = new Row(intLength);
    //increase length for jagged array
    intLength++;
}

As it is now it writes over the current array after each for loop (which is expected). So, I need to index TwoDArray like TwoDArray[i], but as soon as I try to do that then I get this error:

"invalid user-defined conversion from 'Row*' to 'const Row&'."

Note: If I take the line out of the for loop only the first array is made and not until intCol. intLength is increasing because I technically need a jagged array which has got different sizes in each array.

My classes look like this:

    class Row
{
    public:
        //Constructors
        Row();
        Row(int intRows);
        Row(const Row& objPrev);

        //Accessors
        int getNumOfRows();
        int getRowArray(int intRow);

        //Mutators
        void setRowArray(int intRow, int intChange);

        //Destructor
        ~Row();
    private:
        int* OneArray;
        int numOfRows;
}

and

 class Array
{
    public:
        //Constructors
        Array();
        Array(int intRows, int intCols);
        Array(const Array& objPrev);

        //Accessors
        int getNumOfCol();
        Row getTwoDArray(int intCol, int intRow);

        //Mutators
        void setTwoDArray(int intCol, int intRow, int intChageTo);

        //Destructor
        ~Array();
   private:
        Row* TwoDArray;
        int NumOfCol;
}

Any Help or suggestions are appreciated.

To begin with, please don't use pointers and new[] for arrays. If you want an "array" allocated at runtime use std::vector, if you want an array with a fixed size at compiletime use std::array. — Some programmer dude
– Some programmer dude, Commented Sep 28, 2017 at 12:15
Then for your problem, if you want to continue using pointers and new[]: You should use new[] in Array as well. With new Row(intLength) you allocate a single Row object. And you should not do your allocation in a loop. — Some programmer dude
– Some programmer dude, Commented Sep 28, 2017 at 12:16
@Someprogrammerdude I have to use arrays and pointers because that is what the project is about. The reason I called new Row(intLength) is because I have a constructor in Row with those parameters that creates a single array of integers. Which is why I want to call that multiple times (as many as intCols) — LydsE
– LydsE, Commented Sep 28, 2017 at 12:23
"I need to index TwoDArray like TwoDArray[i], but as soon as I try to do that..." which exact peace of code causes the error? — Spock77
– Spock77, Commented Sep 28, 2017 at 12:23
@AlexeyVoytenko where I try and call it like that in the for loop: for(int r=0; r<NumOfCol; r++) { //get the error here: TwoDArray[r] = new Row(intLength); } — LydsE
– LydsE, Commented Sep 28, 2017 at 12:24

Some programmer dude · Accepted Answer · 2017-09-28 12:36:46Z

1

With your loop in Array you allocate a single Row object multiple times, overwriting the pointer in each loop. That leads to a memory leak as only the last will be available through the variable TwoDArray. Also, at the end of the loop all you will have is an "array" of only a single element, the last allocated Row object.

The problem is that you can't do the array allocation using new[] at the same time as you call a specific constructor. You can not do e.g.

TwoDArray = new Row[NumOfCol](intLength);

Instead you have to split the allocation and initialization into two parts:

TwoDArray = new Row[NumOfCol];  // Allocates memory, default constructed Row objects

for (int i = 0; i < NumOfCol; ++i)
{
    TwoDArray[i] = Row(intLength);  // Initialize each element
}

This of course requires you follow the rules of three or five (or zero) for the copying in the loop to work.

edited Sep 28, 2017 at 12:36

answered Sep 28, 2017 at 12:29

Some programmer dude

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1 Comment

LydsE Over a year ago

That makes a lot of sense. Thank you :)

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