What's wrong with this moveable type for std::map?

Question

I want to put a moveable but not copyable type as a value in std::map. Here is some simple code to test the principle.

#include <map>

struct Foo
{
    Foo ();

    Foo (const Foo &) = delete;
    Foo & operator = (const Foo &) = delete;

    Foo (Foo &&) {}
    Foo & operator = (Foo &&) {return *this;}
};

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

    m .insert (std :: make_pair (123, Foo ()));
}

I compile this with g++ test.cpp --std=c++0x (gcc version 4.5.1 on Ubuntu 12.04). There is a big ugly error as shown below. What is the problem?

In file included from /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:66:0,
                 from /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:62,
                 from /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/map:60,
                 from test.cpp:1: test.cpp: In copy constructor ‘std::pair<const int, Foo>::pair(const std::pair<const int, Foo>&)’: /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_pair.h:72:5: instantiated from ‘std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = {const std::pair<const int, Foo>&}, _Val = std::pair<const int, Foo>]’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/ext/new_allocator.h:111:4: instantiated from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*,
_Args&& ...) [with _Args = {const std::pair<const int, Foo>&}, _Tp = std::_Rb_tree_node<std::pair<const int, Foo> >, _Tp* = std::_Rb_tree_node<std::pair<const int, Foo> >*]’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:394:8: 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, Foo>&}, _Key = int, _Val = std::pair<const int, Foo>, _KeyOfValue = std::_Select1st<std::pair<const int, Foo> >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, Foo> >, std::_Rb_tree_node<_Val>* = std::_Rb_tree_node<std::pair<const int, Foo> >*]’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:899:42:   instantiated from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::iterator 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, Foo>, _KeyOfValue = std::_Select1st<std::pair<const int, Foo> >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, Foo> >, std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const int, Foo> >, const std::_Rb_tree_node_base* = const std::_Rb_tree_node_base*]’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:1191:65: instantiated from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_M_insert_unique(const _Val&) [with _Key = int, _Val = std::pair<const int, Foo>, _KeyOfValue = std::_Select1st<std::pair<const int, Foo> >, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, Foo> >]’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_map.h:501:41: instantiated from ‘std::pair<typename std::map<_Key, _Tp, _Compare,
_Alloc>::_Rep_type::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::map<_Key, _Tp, _Compare, _Alloc>::value_type&) [with _Key = int, _Tp = Foo, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, Foo> >, typename std::map<_Key, _Tp, _Compare, _Alloc>::_Rep_type::iterator = std::_Rb_tree_iterator<std::pair<const int, Foo> >, std::map<_Key,
_Tp, _Compare, _Alloc>::value_type = std::pair<const int, Foo>]’ test.cpp:20:43:   instantiated from here test.cpp:7:2: error: deleted function ‘Foo::Foo(const Foo&)’ /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_pair.h:72:5: error: used here In file included from /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/map:60:0,
                 from test.cpp:1: /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h: In constructor ‘std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = {const std::pair<const int, Foo>&}, _Val = std::pair<const int, Foo>]’: /usr/local/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:136:49: note: synthesized method ‘std::pair<const int, Foo>::pair(const std::pair<const int, Foo>&)’ first required here

Answer 1

I'm adding some details for future references: this bug as Quux pointed out was caused in gcc 4.5.1 by a problem in the move constructor for std::pair.

As of today the issue has been fixed and you should either upgrade your compiler version or workaround by using another version of the standard library.

Your code works just fine