What's the purpose of having a separate "operator new[]"?

Question

Looks like operator new and operator new[] have exactly the same signature:

void* operator new( size_t size );
void* operator new[]( size_t size );

and do exactly the same: either return a pointer to a big enough block of raw (not initialized in any way) memory or throw an exception.

Also operator new is called internally when I create an object with new and operator new[] - when I create an array of objects with new[]. Still the above two special functions are called by C++ internally in exactly the same manner and I don't see how the two calls can have different meanings.

What's the purpose of having two different functions with exactly the same signatures and exactly the same behavior?

Answer 1

The operators can be overridden (for a specific class, or within a namespace, or globally), and this allows you to provide separate versions if you want to treat object allocations differently from array allocations. For example, you might want to allocate from different memory pools.

Answer 2

I've had a reasonably good look at this, and to be blunt there's no reason from an interface standpoint.

The only possible reason that I can think of is to allow an optimization hint for the implementation, operator new[] is likely to be called upon to allocate larger blocks of memory; but that is a really, really tenuous supposition as you could new a very large structure or new char[2] which doesn't really count as large.

Note that operator new[] doesn't add any magic extra storage for the array count or anything. It is the job of the new[] operator to work out how much overhead (if any) is needed and to pass the correct byte count to operator new[].

[A test with gcc indicates that no extra storage is needed by new[] unless the type of the array members being constructed have a non-trivial desctructor.]

From an interface and contract standpoint (other than require the use of the correct corresponding deallocation function) operator new and operator new[] are identical.