I’ve been very pleased with Sean Parent’s `stlab::copy_on_write<T>` as a value-semantic mutable alternative to `std::shared_ptr<T>`. It has an explicit `write()

I’ve been very pleased with Sean Parent’s `stlab::copy_on_write<T>` as a value-semantic mutable alternative to `std::shared_ptr<T>`. It has an explicit `write() -> T&` that makes it less magical than some CoW alternatives.