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I have 2 arrays, int a[10][20] and int b[10][20]. I want to create a point to one or the other array and read the data. In one condition I may want a data and the other I might want b data.

Example.

int **ptr;

ptr=&a;  // doesn't work can't figure out how to assign a pointer to and existing 2 dimension array

Please I need the answer not work arounds or alternatives
Posted
Updated 22-Aug-21 6:51am
v4

Pointers to multidimensional arrays in C++ is tricky business. Most often, you have to specify the actual capacity of each dimension, which means one pointer cannot be used for two different sized arrays. If you really want a pointer, I suggest making a single dimension array and then you can just calculate the index to use multiple dimensions. Would go something like this:
C++
int wide = 640; int tall = 480;
int * pArray = new int[wide * tall];
int x; int y; int yOffset; int offset;
for(y = 0; y < tall; y++)
{
	yOffset = y * wide;
	for(x = 0; x < wide; x++)
	{
		offset = yOffset + x;
		// Use pointer arithmetic to assign element at (x,y).
		*(pArray + offset) = CalculateSomething(x, y);
	}
}
 
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v3

Well, they are either the same array, or two different arrays.  You could write a class that swaps which pointer it looks up, but you can't have two arrays that are also the same array, but only sometimes.

 
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Well, I tried, but I'm completely baffled as to why this doesn't work (note the strange error, which is in a comment above the line it occurs at):
C++
// Interface to access array functions (get value, set value).
template<class T>
class IArrayHolder{
public:
	virtual T GetVal(int x, int y) = 0;
	//...Set value...
};

// Class to wrap an array.
template<class T, class S>
class ArrayHolder: public IArrayHolder<S>
{
private:
	T multiArray;
public:
	ArrayHolder(T val){
		// error C2440: '=' : cannot convert from 'int [][20]' to 'int [][20]'
		multiArray = val;
	}
	S GetVal(int x, int y){
		return multiArray[x][y];
	};
};

// Some function that uses multiple multi-dimension arrays.
System::Void SomeFunction(){
	int x[10][20];
	int y[20][30];
	IArrayHolder<int> * xInterface;
	xInterface = new ArrayHolder<int[][20], int>(x);
	IArrayHolder<int> * yInterface;
	yInterface = new ArrayHolder<int[][30], int>(y);
};

// Some function that accepts a 2D array.
System::Void AnotherFunction(IArrayHolder<int> * arrayHolder){
	int val = arrayHolder->GetVal(20, 20);
};

You figure out that error, and you've go your solution. If not, perhaps you should just copy the array values to a 2D vector and use that instead. Also, I don't have much experience with void pointers, but they can apparently point to the address of anything, so you might try working with those.
 
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v2

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