Heap-allocated c arrays don't retain any size information, as pointed out here:

How can i find the size of a dynamically allocated array in C?

However, smart pointers in c++11 have the ability to store memory and manage c-arrays with the array versions using the subscript ([]) operator overload:

std::unique_ptr<int[]> arr(new int[val]);

When this smart pointer goes out of scope, it presumably deallocates the owned block of memory, so it must store the size of the memory block somewhere.

How can I retrieve the size of the c array allocated on arr in the above example, assuming that val is a runtime variable?