Is Self Move Assignment Possible with Rvalue Reference Parameters in C 11?

Self Move Assignment and the Standard Library

In C 11, the standard library provides certain guarantees regarding self move assignment. According to the standard, if a function argument binds to an rvalue reference parameter, the implementation can assume that the parameter is a unique reference to that argument.

Consider the following function:

template<class T>
std::vector<T> selfAssign(std::vector<T> v) {
  v = std::move(v);
  return v;
}

Since the vector argument binds to an rvalue reference parameter, the implementation of std::vector::operator=(vector&& other) can assume that other is a prvalue. Consequently, self-move-assignment is not possible for v in the selfAssign function.

The standard dictates that an rvalue reference parameter can be assumed to be a prvalue, thus precluding the possibility of self-move-assignment in such cases. It is likely that the vector passed to the selfAssign function will be left in a resource-less state or undergo a no-op if it already has 0 capacity.

However, the latest working draft of the standard, N4618, slightly modifies these requirements. Specifically, it states that, if t and rv do not refer to the same object, then after the assignment 't = rv', t may only need to contain the equivalent value of rv prior to the assignment. Regardless, the state of rv remains unspecified after the assignment.

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