operator [] overloading

[darksaga]

Hello qt & c++ fans,

I have some probs overloading the [] operator of my templated grid class.
The template class looks basically like this:

template<class T> class Grid
{
	public:
		Grid(int width, int height) : _pMat(NULL), _iWidth(0), _iHeight(0){
			_iWidth = width;
			_iHeight = height;
			_pMat = createGrid(width, height);
		}
 
		~Grid(void){
			destroyGrid();
		}
 
		Matrix2Dim(const Matrix2Dim<T> &mat){//cpy...}
		Matrix2Dim<T>& operator=(const Matrix2Dim<T> &mat){//cpy...}
 
	private:
		T** createGrid(int width, int height){
			T **pMat = NULL;
			pMat = new T *[height];
			pMat[0] = new T [height * width];
			for (int i = 0; i < height; i++)
				pMat[i] = &pMat[0][(i*width)];
			return pMat;
		}
 
		void destroyGrid(void){
			if(_pMat != NULL){
				delete _pMat[0];
				delete[] _pMat;
				_pMat = NULL;
				_iWidth = _iHeight = 0;
			}
		}
		int _iWidth;
		int _iHeight;
		T** _pMat;
};

template<class T> class Grid
{
	public:
		Grid(int width, int height) : _pMat(NULL), _iWidth(0), _iHeight(0){
			_iWidth = width;
			_iHeight = height;
			_pMat = createGrid(width, height);
		}
		
		~Grid(void){
			destroyGrid();
		}
		
		Matrix2Dim(const Matrix2Dim<T> &mat){//cpy...}
		Matrix2Dim<T>& operator=(const Matrix2Dim<T> &mat){//cpy...}
		
	private:
		T** createGrid(int width, int height){
			T **pMat = NULL;
			pMat = new T *[height];
			pMat[0] = new T [height * width];
			for (int i = 0; i < height; i++)
				pMat[i] = &pMat[0][(i*width)];
			return pMat;
		}
		
		void destroyGrid(void){
			if(_pMat != NULL){
				delete _pMat[0];
				delete[] _pMat;
				_pMat = NULL;
				_iWidth = _iHeight = 0;
			}
		}
		int _iWidth;
		int _iHeight;
		T** _pMat;
};

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now I'd like to access data of the class with the [] operator:

Grid<int> *test = new Grid<int>(2, 2);
test[1][1] = 1;
std::cout << test[1][1] << std::endl;
delete test;

Grid<int> *test = new Grid<int>(2, 2);
test[1][1] = 1;
std::cout << test[1][1] << std::endl;
delete test;

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everything I tried to overload the [] operator failed :/, so maybe somebody here knows how to do it???

T  *operator[](int value) {  
	return ???;
}
 
const T  *operator[](int value) const{  
	return ???;
}

T  *operator[](int value) {  
	return ???;
}

const T  *operator[](int value) const{  
	return ???;
}

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thanks and regards
darksaga

[wysota]

How about:

T *operator[](int value){
  return _pMat[value];
}

T *operator[](int value){
  return _pMat[value];
}

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Just be aware you won't be able to use it as L-value (I think).

[spud]

I think you should check out what the C++ FAQ Lite has to say on the subject. Especially point 13.11


P.S.
That's not to say you shouldn't do it the way you're doing it, but I think you should at least allocate all the memory at once, and then reference it using pointer arithmetic.

[darksaga]

How about:

T *operator[](int value){
  return _pMat[value];
}

T *operator[](int value){
  return _pMat[value];
}

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Just be aware you won't be able to use it as L-value (I think).

if I have a 2x2 int grid ==>

----
|1 2|
|3 4|
 ----

----
|1 2|
|3 4|
 ----

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and use the code above:

Grid<int> *test = new Grid<int>(2, 2);
test->fill();//fill grid that it matches example above
std::cout << test[0][0] << std::endl; // returns an address not a value
std::cout << test[0][1] << std::endl; // returns an address not a value
std::cout << test[1][0] << std::endl; // crash
std::cout << test[1][1] << std::endl; // crash
delete test;

Grid<int> *test = new Grid<int>(2, 2);
test->fill();//fill grid that it matches example above
std::cout << test[0][0] << std::endl; // returns an address not a value
std::cout << test[0][1] << std::endl; // returns an address not a value
std::cout << test[1][0] << std::endl; // crash
std::cout << test[1][1] << std::endl; // crash
delete test;

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but if you look at how I allocate my grid in createGrid() its now wonder that it behaves like that.

i also tried the () operator:

T&  operator()(int height, int width)
{
    return _pMat[height][width];
}
 
const T&  operator()(int height, int width)
{
    return _pMat[height][width];
}

T&  operator()(int height, int width)
{
    return _pMat[height][width];
}

const T&  operator()(int height, int width)
{
    return _pMat[height][width];
}

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but then my compiler won't compile the code...

[spud]

template<class T> 
class Matrix
{
    public:
        Matrix(int width, int height)
			: data(NULL)
			, _iWidth(width)
			, _iHeight(height)
		{
			data = new T[width * height];
        }
        ~Matrix()
		{
            delete [] data;
        }
        T* operator[](int row) // index starts with zero
		{  
			return data + row*_iWidth;
		}
        T const* operator[](int row)const
		{  
			return data + row*_iWidth;
		}
        T& operator()(int row, int col) // indexs start with one
		{  
			return *(data + (row-1)*_iWidth + col-1);
		}
        const T& operator()(int row, int col)const
		{  
			return *(data + (row-1)*_iWidth + col-1);
		}
    private:
        int _iWidth;
        int _iHeight;
        T* data;
};

template<class T> 
class Matrix
{
    public:
        Matrix(int width, int height)
			: data(NULL)
			, _iWidth(width)
			, _iHeight(height)
		{
			data = new T[width * height];
        }
        ~Matrix()
		{
            delete [] data;
        }
        T* operator[](int row) // index starts with zero
		{  
			return data + row*_iWidth;
		}
        T const* operator[](int row)const
		{  
			return data + row*_iWidth;
		}
        T& operator()(int row, int col) // indexs start with one
		{  
			return *(data + (row-1)*_iWidth + col-1);
		}
        const T& operator()(int row, int col)const
		{  
			return *(data + (row-1)*_iWidth + col-1);
		}
    private:
        int _iWidth;
        int _iHeight;
        T* data;
};

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[darksaga]

thanks @ spud

especially for this link.

I had some time to read through the article, now everything works as desired ...