Why are move semantics for a class containing a st

问题:

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!

回答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).

回答2:

Clang 3.0 is C++98, Clang 3.1 compiles this just fine (with libc++):

~/blargh $ cat t.cpp 
#include <sstream>

int main(){
  auto get_s = []{ return std::stringstream("hi"); };
  auto s = get_s();
}
~/blargh $ clang -v
clang version 3.1 (trunk 152621)
Target: x86_64-unknown-linux-gnu
Thread model: posix
~/blargh $ clang++ -std=c++0x -stdlib=libc++ -Wall -pedantic t.cpp 
~/blargh $

Your example comples just fine too, so I guess it's libstdc++ that's broken in that regard.

回答3:

You are likely using a bad version of the library. There is no reason why your code should be invalid.

By the way, self-assignment checking assignment operators are bad.

Edit: Curiously, Visual Studio also seems to believe that basic_stringstream has no move constructor. All streams should be movable in C++11. Perhaps this is a Standard defect, or implementing the move constructor requires a C++11 feature neither yet support. Conceptually, there's no reason why it should fail, but I can't actually find anything to support that.