How do I call the original "operator new" if I have overloaded it?

Question

Suppose I need to overload global ::operator new() for storing extra data with each allocated object. So basically it would work this way:

for each call to global ::operator new() it will take the object size passed and add the size of extra data
it will allocate a memory block of size deduced at previous step
it will offset the pointer to the part of the block not occupied with extra data and return that offset value to the caller

::operator delete() will do the same in reverse - shift the pointer, access extra data, deallocate memory.

Now the question is how do I allocate memory? Of course I can call malloc() or some platform-specific function (that's how it is usually done). But normally when I need to allocate raw memory in C++ I call ::operator new(). Can I call the original ::operator new() to do the memory allocation from inside my overloaded global ::operator new()?

Do you mean overloaded or replaced? If overloaded, just call the original operator new by using the correct set of arguments. If replaced, then no you can't but I believe this is why malloc is guaranteed not to use operator new in its implementation (i.e. no unintentional loops when operator new is replaced) so you probably should be using malloc. — CB Bailey
@CharlesBailey You wrote your answer as a comment! for shame! — lmat - Reinstate Monica

Potatoswatter Potatoswatter · Accepted Answer · 2010-11-09T13:50:52

You can't access them because it isn't really overloading, it's replacement. When you define your own ::operator new, the old one goes away. That's pretty much that.

Essentially, you need to call malloc from a custom ::operator new. Not only that, but also follow the directions in 18.4.1.1/4 to properly handle errors:

Default behavior:

— Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library function malloc is unspecified.

— Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument to set_new_handler() was a null pointer, throw bad_alloc.

— Otherwise, the function calls the current new_handler (18.4.2.2). If the called function returns, the loop repeats.

— The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return.

How do I call the original "operator new" if I have overloaded it?

1 Answers