This is where Rust is worth mentioning: its Rc smart pointer will statically prevent multiple threads from accessing the inner value, while its Arc (atomic reference counting) smart pointer will let you share the value while using atomic operations to adjust the refcount, same as C++'s shared_ptr. Rust's move semantics by-default also mean fewer refcount adjustments overall, since you can transfer ownership instead.

There's also a neat new copy-on-write smart pointer in the stdlib, though I have no experience with it yet: http://doc.rust-lang.org/std/borrow/enum.Cow.html