Why doesn't `std::stringstream::stringstream(std::string&&)` exist?

Question

I was hoping stringstream has a constructor that steals its initial content from a string&&. Do such inter-species \"move constructors\" genera

Answer 1

Why doesn't std::stringstream::stringstream(std::string&&) exist?

This is due to std::stringstream's internal buffer, rdbuf.

rdbuf, (type std::string_buf), doesn't support non-copy access as per the motivation in proposal, p0408r4:

... there is no non-copying access to the internal buffer of a basic_stringbuf which makes at least the obtaining of the output results from an ostringstream inefficient, because a copy is always made

However, there's already a plan to support std::string move in stringsteam's constructor:

explicit basic_ostringstream(
   basic_string&& str,
   ios_base::openmode which = ios_base::out,
   const Allocator& a = Allocator());

AND move str()

template
void str(basic_string&& s);