C++ doesn't tell you the size of a dynamic array. But why?

Question

I know that there is no way in C++ to obtain the size of a dynamically created array, such as:

int* a; a = new int[n]; 

What I would like to know is: Why? Did people just forget this in the specification of C++, or is there a technical reason for this?

Isn't the information stored somewhere? After all, the command

delete[] a; 

seems to know how much memory it has to release, so it seems to me that delete[] has some way of knowing the size of a.

Answer 1

It's a follow on from the fundamental rule of "don't pay for what you don't need". In your example delete[] a; doesn't need to know the size of the array, because int doesn't have a destructor. If you had written:

std::string* a; a = new std::string[n]; ... delete [] a; 

Then the delete has to call destructors (and needs to know how many to call) - in which case the new has to save that count. However, given it doesn't need to be saved on all occasions, Bjarne decided not to give access to it.

(In hindsight, I think this was a mistake ...)

Even with int of course, something has to know about the size of the allocated memory, but:

• Many allocators round up the size to some convenient multiple (say 64 bytes) for alignment and convenience reasons. The allocator knows that a block is 64 bytes long - but it doesn't know whether that is because n was 1 ... or 16.

• The C++ run-time library may not have access to the size of the allocated block. If for example, new and delete are using malloc and free under the hood, then the C++ library has no way to know the size of a block returned by malloc. (Usually of course, new and malloc are both part of the same library - but not always.)