boost :: multi_array索引到内存偏移量的转换

Question

I have three-dimensional boost::multi_array object. I would like to convert an absolute offset from the origin to a multi-dimensional index and vice versa. Is there an easy way to do so using boost's built-in facilities, or I need to compute it myself given the array's shape and strides?

Answer 1

Of course you can use some library facility.

For the offset, you can easily use pointer arithmetics¹:

size_t offset = &ma[1][2][3] - ma.data();
std::cout << "Offset of ma[1][2][3]: " << offset << "\n";

The documentation divulges a more elaborate way to get the element address: link

 This expression accesses a specific element of a.index_list is the unique set of indices that address the element returned. It is equivalent to the following code (disregarding intermediate temporaries): // multiply indices by strides std::transform(index_list.begin(), index_list.end(), a.strides(), tmp.begin(), std::multiplies<index>()), // add the sum of the products to the origin *std::accumulate(tmp.begin(), tmp.end(), a.origin()); 

The inverse less simple. I couldn't find a direct way.

¹ note: use std::addressof to guard against elements overload operator& in generic code