c++ - 作者所说的“让对象处于可分配状态”是什么意思？

Question

以下是Thomas Becker 先生在本文结尾处的一个片段：

X& X::operator=(X&& rhs)
{

  // Perform a cleanup that takes care of at least those parts of the
  // destructor that have side effects. Be sure to leave the object
  // in a destructible and **assignable state**.

  // Move semantics: exchange content between this and rhs

  return *this;
}

作者上面所说的“让对象处于可分配状态”是什么意思？