invariants

I've been trying to wrap my head around how move semantics in C++11 are supposed to work, and I'm having a good deal of trouble understanding what conditions a moved-from object needs to satisfy. Looking at the answer here doesn't really resolve my question, because can't see how to apply it to pimpl objects in a sensible way, despite arguments that move semantics are perfect for pimpls . The easiest illustration of my problem involves the pimpl idiom, like so: class Foo { std::unique_ptr<FooImpl> impl_; public: // Inlining FooImpl's constructors for brevity's sake; otherwise it // defeats the