Safely moving a C++ object

Question

I’ve heard some words of warning against shipping an object to another memory location via memcpy, but I don’t know the specific reasons. Unless its contained members do tricky things that depend on memory location, this should be perfectly safe … or not?

EDIT: The contemplated use case is a data structure like a vector, which stores objects (not pointers to objects) in a continuous chunk of memory (i.e. an array). To insert a new object at the n-th position, all objects starting at position n and beyond will need to be moved to make room for the object to be inserted.

Answer 1

One primary reason why you should not do this is destructors. When you memcpy a C++ object to another location in memory, you will end up with 2 versions of the object in memory for which only 1 constructor has been run. This will destroy the resource freeing logic of pretty much every single C++ class out there.

Answer 2

It's not allowed by the language specification. It is undefined behavior. That is, ultimately, what's wrong with it. In practice, it tends to mess with virtual function calls, and it means the destructor will be run twice (and more often than the constructors), member objects are shallow copied (so if, for example, if you try this stunt with a std::vector, it blows up, as multiple objects end up pointing to the same internal array.)

The exception is POD types. They don't have (copy) constructors, destructors, virtual functions, base classes or anything else that might cause this to break, so with those, you're allowed to use memcpy to copy them.