Why are move semantics for a class containing a std::stringstream causing compiler errors?

Question

How can I make this simple class movable? What I thought was correct just produces a wall of errors...

#include <iostream> #include <sstream> #include <utility>  class message {     public:         message() = default;          // Move constructor         message( message &&other ) :             stream_( std::move( other.stream_ ) ) // Nope         {}          // Move assignment         message &operator=( message &&other )         {             if ( this != &other )             {                 stream_ = std::move( other.stream_ ); // Nope #2             }             return *this;         }      private:         message( const message & ) = delete;         message &operator=( const message & ) = delete;          std::stringstream stream_;         // Other member variables omitted };  int main() {     message m;      return 0; } 

Compile:

$ g++ --version g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 $ g++ -Wall -Wextra -std=c++0x move.cpp -o move 

... and get a wall of errors about the copy assignments being called for various base classes of the stringstream.

move.cpp: In constructor ‘message::message(message&&)’: move.cpp:12:40: error: use of deleted function ‘std::basic_stringstream<char>::basic_stringstream(const std::basic_stringstream<char>&)’ In file included from move.cpp:2:0: /usr/include/c++/4.6/sstream:483:11: error: ‘std::basic_stringstream<char>::basic_stringstream(const std::basic_stringstream<char>&)’ is implicitly deleted because the default definition would be ill-formed: /usr/include/c++/4.6/sstream:483:11: error: use of deleted function ‘std::basic_iostream<char>::basic_iostream(const std::basic_iostream<char>&)’ In file included from /usr/include/c++/4.6/iostream:41:0,                  from move.cpp:1: /usr/include/c++/4.6/istream:774:11: error: ‘std::basic_iostream<char>::basic_iostream(const std::basic_iostream<char>&)’ is implicitly deleted because the default definition would be ill-formed: /usr/include/c++/4.6/istream:774:11: error: use of deleted function ‘std::basic_istream<char>::basic_istream(const std::basic_istream<char>&)’ /usr/include/c++/4.6/istream:57:11: error: ‘std::basic_istream<char>::basic_istream(const std::basic_istream<char>&)’ is implicitly deleted because the default definition would be ill-formed: /usr/include/c++/4.6/istream:57:11: error: use of deleted function ‘std::basic_ios<char>::basic_ios(const std::basic_ios<char>&)’ [SNIP] 

... This goes on for several pages.

What's wrong with my code?

Update 1: Clang 3.0 fails with similar results.
Update 2: g++ 4.7 fails also.
Update 3: Using the answers as a guide, I found this: c++11 status in libstdc++ - "27.5 Iostreams base classes: Missing move and swap operations on basic_ios." Curses!

Answer 1

Update

It is required to work according to the C++11/14 standards. GCC 5.0 gets it right, and the below mentioned bug is RESOLVED. Thank you GCC team!

Original answer

It's a missing feature in gcc (or as Xeo points out libstdc++) as of yet.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316

According to the standard,

typedef basic_stringstream<char> stringstream; 

and from 27.8.5, there is a public move constructor

basic_stringstream(basic_stringstream&& rhs); 

I can confirm the problem with gcc 4.7 -std=c++11, ubuntu 12.04.

std::stringstream a; std::stringstream b=std::move(a); 

reading the include file include/std/sstream I find no move constructor or any mentioning of C++0x or C++11. (compare with std::string which does work.)

Adding a (mock) move constructor:

basic_stringstream(basic_stringstream&& rhs){} 

reduces the error to only tree lines, but

use of deleted function ‘std::basic_stringstream<char>::basic_stringstream(    const std::basic_stringstream<char>&)’ 

remains.

Workaround

Use a std::unique_ptr<std::stringstream> instead. Best is to initialize it with make_unique which comes with c++14, or take it for example from my blog cpp11style-no-new-delete (that is an earlier version, but it will work fine for this purpose).