Question

Well I don’t think that it’s really important but since the program has to store the length because of delete[] anyway, Why can’t we get this “stored information” ?

Answer 1

The implementation only needs to store the length, and typically only does, if the type is not trivially destructible (i.e., it needs to generate calls to a destructor) and the array was created with the new[] operator.

Since that property of the arrayed type bears no relation to the size of the array, it is more elegant simply to call the length “cookie” a private implementation detail.

To get the length of a complete array object (not a mere pointer), you can use std::extent< decltype( arr ) >::value or std::end( arr ) - std::begin( arr ).

Using new[] with a class with a destructor is a code smell. Consider std::vector instead. The overhead vs raw new[] (considering all bytes that need to be allocated, wherever they are) is one pointer’s worth of bytes, and the benefits are innumerable.