Why is it not possible to access the size of a new[]'d array?

In a typical implementation the size of dynamic memory block is somehow stored in the block itself - this is true. But there's no standard way to access this information. (Implementations may provide implementation-specific ways to access it). This is how it is with malloc/free, this is how it is with new[]/delete[].

In fact, in a typical implementation raw memory allocations for new[]/delete[] calls are eventually processed by some implementation-specific malloc/free-like pair, which means that delete[] doesn't really have to care about how much memory to deallocate: it simply calls that internal free (or whatever it is named), which takes care of that.

What delete[] does need to know though is how many elements to destruct in situations when array element type has non-trivial destructor. And this is what your question is about - the number of array elements, not the size of the block (these two are not the same, the block could be larger than really required for the array itself). For this reason, the number of elements in the array is normally also stored inside the block by new[] and later retrieved by delete[] to perform the proper array element destruction. There are no standard ways to access this number either.

(This means that in general case, a typical memory block allocated by new[] will independently, simultaneously store both the physical block size in bytes and the array element count. These values are stored by different levels of C++ memory allocation mechanism - raw memory allocator and new[] itself respectively - and don't interact with each other in any way).

However, note that for the above reasons the array element count is normally only stored when the array element type has non-trivial destructor. I.e. this count is not always present. This is one of the reasons why providing a standard way to access that data is not feasible: you'd either have to store it always (which wastes memory) or restrict its availability by destructor type (which is confusing).

To illustrate the above, when you create an array of ints

int *array = new int[100];

the size of the array (i.e. 100) is not normally stored by new[] since delete[] does not care about it (int has no destructor). The physical size of the block in bytes (like, 400 bytes or more) is normally stored in the block by the raw memory allocator (and used by raw memory deallocator invoked by delete[]), but it can easily turn out to be 420 for some implementation-specific reason. So, this size is basically useless for you, since you won't be able to derive the exact original array size from it.

You most likely can access it, but it would require intimate knowledge of your allocator and would not be portable. The C++ standard doesn't specify how implementations store this data, so there's no consistent method for obtaining it. I believe it's left unspecified because different allocators may wish to store it in different ways for efficiency purposes.

It makes sense, as for example the size of the allocated block may not necessarily be the same size as the array. While it is true that new[] may store the number of elements (calling each elements destructor), it doesn't have to as it wouldn't be required for a empty destructor. There is also no standard way (C++ FAQ Lite 1, C++ FAQ Lite 2) of implementing where new[] stores the array length as each method has its pros and cons.

In other words, it allows allocations to as fast an cheap as possible by not specifying anything about the implementation. (If the implementation has to store the size of the array as well as the size of the allocated block every time, it wastes memory that you may not need).