Behavior of operator[] on 1D and 2D arrays in C++

Question

I've enocuntered a very strange exception when writting code in C++Amp. I define two concurrency::array objects as follows:

concurrency::array<float, 2> img_amp_data(11, 11, image_data.begin()); 
concurrency::array<float> a_amp_result(121, empty_vec.begin()); 

When I want to access the elements of the first of them

std::cout << img_amp_data[0][0] << std::endl; 

everything runs properly, but when I want to access the second one

std::cout << a_amp_result[0] << std::endl; 

I get a following exception:

Exception: (The array is not accessible on CPU)

It is a very odd situatuon because I can access 2+ dimensional arrays and it is forbidden to access only one dimensional array? Any ideas?

Answer 1

You are running into a syntax quirk.

For an array of rank larger than 1, operator[] with integral parameter returns an array_view referring to the slice of the original array. The subsequent operator[] operates on the array_view object, which is allowed on the host -- and in your case causes an implicit data copy before returning a reference to the element.

For an array of rank 1 operator[] with integral parameter, or operator[] with index parameter for any array, return a reference to the element. Both are allowed only on the location where the array is resident - by default it is the default accelerator_view, however you can request to create the array in the CPU memory as well.

Unless you want to have fine-grained control on the data movement between the host and the accelerator_view, it is suggested to use the array_view type ubiquitously.