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!
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!
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.
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).