我没有看到 std :: stringstream 构造函数接受 std :: string

没错。即使 str 设置程序也不会使用移动语义，所以将一个字符串移动到 stringstream 是不可能的（不是在当前标准，但希望在未来）。

In the c++ reference I do not see a std::stringstream constructor accepting rvalue reference of std::string. Is there any other helper function to move string to stringstream without an overhead or is there a particular reason behind making such limitation?

I do not see a std::stringstream constructor accepting rvalue reference of std::string

That's right. Even the str setter doesn't utilize move semantics, so moving a string into stringstream is not possible (not in the current standard, but hopefully in the future).