我有一个包含
std::unique_ptr<>
的类,我想将该类的实例放入std::map<>
中。我认为引入C++移动语义的原因之一是可以将像unique_ptrs
这样的东西放入标准容器中(在向量的情况下确实有效)。但我觉得std::map<>
不喜欢这个想法。为什么呢?#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;
}
谢谢。
-- 编辑
为了更清楚,确切的错误信息如下:
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
看起来这是与std::pair<>
有关的问题,但是当它单独使用时,它完全正常:
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;
}
虽然这显然并不意味着它不能被滥用:
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;
}
这可能是std::map<>
内部发生的情况。
有线索吗?
-- 编辑
检查错误消息后,发现这实际上是TDM-GCC 4.4.1的std::map<>
实现存在问题。它似乎没有像std::vector<>::push_back(value_type&&)
那样支持移动语义插入方法。
现在该怎么办?
<memory>
中使用unique_ptr。这可能是你看到该消息的原因。在这里,unique_ptr被很好地使用了。只是std::map有些混乱。 - Gui Prámap
不能处理非可复制类型,那么仅解决这个问题是没有帮助的。迟早你会在其他地方遇到其他问题。 - Johannes Schaub - litb