Consider following pattern, stripped of details for simplicity:
QString str = /* expression */
/* some actions with str */
foo(std::move(str));
str = /* expression */
/* some actions with str */
foo(std::move(str));
Does reuse of str yield defined behaviour in Qt 4.8.6? Qt 5.x?
A move operation leaves the moved-from object in a "valid but unspecified state":
16.5.5.16 Moved-from state of library types [lib.types.movedfrom]
Objects of types defined in the C ++ standard library may be moved from. Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.
This statement, from the C++ standard, refers to the classes from the C++ library, as noted. However, it is a fairly easy bet that the Qt library seeks to comply to the same standards here, and this applies to the Qt library as well. It's a very safe bet to make (and it happens to be true, too).
"Valid but unspecified" state means that the moved-from object is still a valid, well-formed object. Because it's unspecified you have no guarantees, whatsoever, that if you call its size() method that you'll get any particular value. It could be 0 (probably), or it could be something else. However, since it's a valid object, whatever you get from a size() of a moved-from QString, if you then proceed and check what's in the string, you'll see the same exact number of character in there.
And, since it's a valid QString object you are free to completely replace its contents by assigning something to it, leaving the object is a fully-specified, valid state.