It does not necessarily add a branch to the lookup, look more closely: all current data exists in buf in its entirety. It is duplicated into next_buf. This mig

It does not necessarily add a branch to the lookup, look more closely: all current data exists in buf in its entirety. It is duplicated into next_buf. This might be problematic depending on T in C++ as it would need to invoke copy constructors. In Rust all types can trivially be memcpy'd into another location if you own it, so this design would be entirely valid (as long as next_buf is not publicly accessible at all, and we do not provide mutable access to the data). EDIT: after thinking a bit more, due to interior mutability you can't provide access at all, or you would need to invoke a branch like you said in Rust. If you want to provide mutable access to a piece of data and/or avoid copy constructors in favor of move constructors, then, yes you would need to do a branch.