Generating 2D array

Ok, Still have issues with pointers.

I want to be able to dynamically allocate for a 2d array. I was wondering what was necessary, this was my first attempt:

namespace Matrix{
template <class T> T * GenerateMatrix (int rows, int columns)throw(std::string){
    if (rows>15||columns>15) {
        throw std::string ("Too many rows or columns in Generate Matrix");
    }
    T** array = new T*[rows];
    for(int i=0; i<columns;++i){
        array[i] = new T[columns];
    }
    for (int i=0;i<rows;i++){
        for (int j=0;j<columns;j++){
            array[i][j]=0;
        }
    }
    return &&array;
}
int main (){
    double * array = Matrix::GenerateMatrix<double>(3,5);
}

Now I think the problem is the array is deallocated once GenerateMatrix is out of scope. The error when I try to compile is:
|18|error: invalid conversion from 'void*' to 'double*'|
So if you could point me in the right direction as how to generate a 2d array dynamically I would appreciate it.

I guess I could try:

class Matrix {
    int *rows, *columns;
  public:
    Matrix (int,int);
    ~Matrix ();
    double** ary = new double*[rows];
    for(int i = 0; i < rows; ++i){
        ary[i] = new double[colums];
    }
    };

Matrix::Matrix (int a, int b) {
  rows = new int;
  columns = new int;
  *rows = a;
  *columns = b;
}

Matrix::~Matrix () {
  delete rows;
  delete columns;
}

but man, that gives the most error messages I've ever seen

Last edited on

coder777 (8444)

due to your unfavorable format of braces you simply forgot one + some mistakes:

namespace Matrix{
template <class T> T ** GenerateMatrix (int rows, int columns)throw(std::string){ // NOTE: T**
    if (rows>15||columns>15) {
        throw std::string ("Too many rows or columns in Generate Matrix");
    }
    T** array = new T*[rows];
    for(int i=0; i<columns;++i){
        array[i] = new T[columns];
    }
    for (int i=0;i<rows;i++){
        for (int j=0;j<columns;j++){
            array[i][j]=0;
        }
    }
    return array; // NOTE: no &&
} // NOTE
}
int main (){
    double ** array = Matrix::GenerateMatrix<double>(3,5); // NOTE: **
}

Disch (13742)

Also note the idea of passing back an allocated pointer is a bad idea -- as it makes memory leaks much more likely and makes the code much harder to use.

I'd go with your new idea of putting it in a class and have the appropriate deleting in the dtor. Although in your Matrix class, there's no reason to allocate 'rows' and 'columns' dynamically like that.

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Generating 2D array