Can't create map of MoveConstructibles

Question

Hello,

I have a class containing a std::unique_ptr<> and I want to put instances of this class inside of an std::map<>. I thought one of the things that motivated the introduction of move semantics to C++ was the possibility of putting things like unique_ptrs inside standard containers (and that really works, in the case of vectors). But it seems to me that std::map<> doesn't like this idea. Why is this so?

#include <map>
#include <memory>

int main()
{
    std::map<int,std::unique_ptr<int>> map;

    // error on the line that follows (use of disabled lvalue copy constructor)
    map.insert(std::make_pair(1,std::unique_ptr<int>(new int(2))));

    return 0;
}

Thanks.

-- edit

Just to be more clear, the exact error messages are:

mingw32-g++.exe --std=gnu++0x   -ID:\CodeEnv\Libraries\Boost  -c D:\CodeEnv\CodeMess\Untitled1.cpp -o D:\CodeEnv\CodeMess\Untitled1.o
mingw32-g++.exe  -o D:\CodeEnv\CodeMess\Untitled1.exe D:\CodeEnv\CodeMess\Untitled1.o   
In file included from c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_algobase.h:66,
                 from c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:62,
                 from c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/map:60,
                 from D:\CodeEnv\CodeMess\Untitled1.cpp:1:
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/unique_ptr.h: In copy constructor 'std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >::pair(const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&)':
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_pair.h:68:   instantiated from 'std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&, _Val = std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/ext/new_allocator.h:111:   instantiated from 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&, _Tp = std::_Rb_tree_node<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:394:   instantiated from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&, _Key = int, _Val = std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >, _KeyOfValue = std::_Select1st<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:881:   instantiated from 'std::_Rb_tree_iterator<_Val> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_(const std::_Rb_tree_node_base*, const std::_Rb_tree_node_base*, const _Val&) [with _Key = int, _Val = std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >, _KeyOfValue = std::_Select1st<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:1177:   instantiated from 'std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = int, _Val = std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >, _KeyOfValue = std::_Select1st<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_map.h:500:   instantiated from 'std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [with _Key = int, _Tp = std::unique_ptr<int, std::default_delete<int> >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, std::unique_ptr<int, std::default_delete<int> > > >]'
D:\CodeEnv\CodeMess\Untitled1.cpp:7:   instantiated from here
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/unique_ptr.h:214: error: deleted function 'std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = int, _Tp_Deleter = std::default_delete<int>]'
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_pair.h:68: error: used here
In file included from c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/map:60,
                 from D:\CodeEnv\CodeMess\Untitled1.cpp:1:
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h: In constructor 'std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&, _Val = std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >]':
c:\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:136: note: synthesized method 'std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >::pair(const std::pair<const int, std::unique_ptr<int, std::default_delete<int> > >&)' first required here 

It would look like that's a problem with std::pair<>, but it works perfectly fine when used alone:

int main()
{
    std::pair<int,std::unique_ptr<int>> pair;

    pair = std::make_pair(1,std::unique_ptr<int>(new int(2))); // no errors
    return 0;
}

Although that obviously doesn't mean it can't be misused:

int main()
{
    std::pair<int,std::unique_ptr<int>> pair1,pair2;

    pair1 = std::make_pair(1,std::unique_ptr<int>(new int(2))); // no errors
    pair2 = pair1; // BOOM ! Copy from lvalue
    return 0;
}

which probably is what happens inside of std::map<>.

Clues?

-- edit

Reviewing the error messages, it really happens to be a problem with TDM-GCC 4.4.1's std::map<> implementation. It doesn't seem to feature a move semantics-aware insertion method like std::vector<>::push_back(value_type&&).

What to do now?

Answer 1

What compiler are you using? It compiles fine with VS10 beta 2.

---

Edit: Compiler in question is GCC

This appears to be a bug in the STL implementation, since GCC is open source you can submit a patch fixing this issue.
Luckily the fix is in the STL code and not the compiler code so it shouldn't be too difficult.

In the meantime you can enter your fix in the local pair/map header files.

Answer 2

I ran into the same problem. I was able to add the required functionality without too much effort by patching bits/stl_map.h and bits/stl_tree.h. To get your particular example to work, you basically just have to add an insert(value_type&&) method to the map class. It is essentially the same as insert(const value_type&), but using std::move() where appropriate. You have to add the right support methods as well, but you can use the compile errors to tell you what is needed.

This issue has been reported to the GCC team (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44436) and someone is working on it, so hopefully we will see support for move constructibles in associative and unordered containers soon.