Pointer to 2D array as function parameter

Question

This C++ code worked for me so far:

Main.cpp:

unsigned __int16** image_data; image_data = Grabber->get_image_data(1);

interface.cpp:

unsigned __int16** Grabber::get_image_data(int image_num) { unsigned __int16 **pixel_values = 0; pixel_values = new unsigned __int16*[height]; for (int h = 0; h < height; h++) { pixel_values[h] = new unsigned __int16[width]; for (int w = 0; w < width; w++) { pixel_values[h][w] = ...; } } return pixel_values; }

But now I would like to pass the array as pointer to the function. I tried it like in following code, but it doesnt work anymore.

Main.cpp:

unsigned __int16** image_data; Grabber->get_image_data(1, &image_data);

Interface.cpp:

int Grabber::get_image_data(int image_num, unsigned __int16*** image_data) { *image_data = new unsigned __int16*[height]; for (int h = 0; h < height; h++) { *image_data[h] = new unsigned __int16[width]; for (int w = 0; w < width; w++) { *image_data[h][w] = ...; } } return 0; }

Are there any errors in reasoning by me?

#define dosen't work for me, you are getting compiler error? — Andrzej Budzanowski
– Andrzej Budzanowski, Commented Sep 26, 2016 at 9:24
I've edited out the dots; if they contain important code, you should include them. If not, it doesn't matter and you can just leave them out. — Bartek Banachewicz
– Bartek Banachewicz, Commented Sep 26, 2016 at 9:26
You shouldn't store the image data like that; it's by far easier and better to utilize a flat buffer (that is, __int16*), and manually calculate the index. Better still, use std::vector<int16_t>, which utilizes the standard typedef and automatically manages its memory. — Bartek Banachewicz
– Bartek Banachewicz, Commented Sep 26, 2016 at 9:28

molbdnilo · Accepted Answer · 2016-09-26 09:26:16Z

*image_data[h] means *(image_data[h]) (that is, image_data[h][0]), not (*image_data)[h] (or image_data[0][h]).
The latter is what you want.

It's easier to get it right if you introduce a local variable:

int ..::get_image_data(int image_num, unsigned __int16*** image_data) { ... unsigned __int16** data = new unsigned __int16*[height]; *image_data = data; for (int h = 0; h < height; h++) { data[h] = new unsigned __int16[width]; for (int w = 0; w < width; w++) { data[h][w] = ...; } } return 0; }

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