Converting pointer to 2D array

Question

I'm calling C++ method from C# code and passing the pointer of first element of a 2D array, and the dimensions of the array to the C++ function:

C# code

fixed (bool* addressOfMonochromeBoolMatrixOfBitmap = &monochromeBoolMatrixOfBitmap[0, 0]
{             
    NativeGetUniquenessMatrixOfBitmap(addressOfMonochromeBoolMatrixOfBitmap, Width, Height);         
}

C++ code

extern "C" __declspec(dllexport) void NativeGetUniquenessMatrixOfBitmap ( bool* addressOfMonochromeBoolMatrixOfBitmap, 
    int boolMatrixWidth, int boolMatrixHeight)
{
}

In the C++ code I want to cast the bool* pointer to 2D array, in order to access the elements using conventional array syntax: someArray[1][4]. I've tried this code:

bool (*boolMatrix)[boolMatrixWidth][boolMatrixHeight] = (bool (*)[boolMatrixWidth][boolMatrixHeight])addressOfMonochromeBoolMatrixOfBitmap;

But it doesn't compile giving the message "expected constant expression".

Please share any ideas. Thanks.

Al Kepp · Accepted Answer · 2011-10-17 21:35:41Z

2

In C++ 2D array only one array dimension can be from variable, the other one must be constant. So you have to keep the pointer and compute the address of each pixel manually.

And I am affraid you have to swap height and width, i.e. correct is probably [height][width]. If it is so, then height can be variable, but width must be constant. If it isn't constant, you have to keep the bool* pointer and compute address of each row like row = address + width*y and then you can use row[x] to access particular items in the row.

//we prepare row pointer to read at position x,y 
bool *row = addressOfMonochromeBoolMatrixOfBitmap + boolMatrixWidth * y;

//example: read from x,y
bool value_at_xy = row[x];

//example: move to next row
row += boolMatrixWidth;

edited Oct 17, 2011 at 21:35

answered Oct 17, 2011 at 21:29

Al Kepp

5,9803 gold badges30 silver badges52 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Converting pointer to 2D array

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related