Is there any heap compaction in C++?

Question

I have a notion that C++ runtime doesn't do any heap compaction which means that the address of an object created on heap never changes. I want to confirm if this is true and also if it is true for every platform (Win32, Mac, ...)?

Answer 1

The C++ standard says nothing about a heap, nor about compaction. However, it does require that if you take the address of an object, that address stays the same throughout the object's lifetime.

A C++ implementation could do some kind of heap compaction and move objects around behind the scenes. But then the "addresses" it return to you when you use the address-of operator, are not actually memory addresses but some other kind of mapping.

In other words, yes, it is safe to assume that addresses in C++ stay the same while the object you're taking the address of lives.

What happens behind the scenes is unknown. It is possible that the physical memory addresses change (although common C++ compilers wouldn't do this, it might be relevant for compilers targeting various forms of bytecode, such as Flash), but the addresses that your program sees are going to behave nicely.