地圖/ unordered_map與不可移動的，缺省構造的值類型

Question

更新：它的出現，在23.5.4.3 here的m[1]版本應該是有效的

UPDATE2：m[1]正在與gcc4.9.1

具體而言，std::mutex。假設我想要一個std::unordered_map<int, std::mutex>。這可能嗎？

我似乎無法添加mutex到map沒有在一定程度上已刪除的拷貝構造函數引發錯誤，無論是的mutex或持有互斥實習生std::pair的。

m.emplace(1); 
m.emplace(1, {}); 
m[1]; 

都無法編譯。我真正想要的是mutex在map的內部構建。

我寧願不使用std::unique_ptr<std::mutex>，但我知道這是可能的，這是我的最佳選擇。

新增：使用operator[]一個完整的例子在gcc和鐺失敗對我來說

#include <unordered_map> 
#include <mutex> 

int main() { 
    std::unordered_map<int, std::mutex> m; 
    m[1]; 

    return 0; 
} 

錯誤從clang3.5

/usr/bin/../lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/stl_pair.h:126:35: error: call to deleted constructor of 'std::mutex' 
     : first(std::forward<_U1>(__x)), second(__y) { } 
             ^ ~~~ 
... 

f.cpp:6:6: note: in instantiation of member function 'std::__detail::_Map_base<...>::operator[]' requested here 
    m[1]; 
    ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/mutex:163:5: note: 'mutex' has been explicitly marked deleted here 
    mutex(const mutex&) = delete; 
    ^

，並從GCC 4.7.2最相關的部分

In file included from /usr/include/c++/4.7/utility:72:0, 
       from /usr/include/c++/4.7/unordered_map:38, 
       from f.cpp:1: 
/usr/include/c++/4.7/bits/stl_pair.h: In instantiation of ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, const _T2&) [with _U1 = int; <template-parameter-2-2> = void; _T1 = int; _T2 = std::mutex]’: 
/usr/include/c++/4.7/bits/stl_pair.h:273:72: required from ‘constexpr std::pair<typename std::__decay_and_strip<_T1>::__type, typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = std::mutex; typename std::__decay_and_strip<_T2>::__type = std::mutex; typename std::__decay_and_strip<_T1>::__type = int]’ 
/usr/include/c++/4.7/bits/hashtable_policy.h:463:24: required from ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](_Key&&) [with _Key = int; _Pair = std::pair<const int, std::mutex>; _Hashtable = std::_Hashtable<int, std::pair<const int, std::mutex>, std::allocator<std::pair<const int, std::mutex> >, std::_Select1st<std::pair<const int, std::mutex> >, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>; std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = std::mutex]’ 
f.cpp:6:8: required from here 
/usr/include/c++/4.7/bits/stl_pair.h:126:45: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’ 
In file included from f.cpp:2:0: 
/usr/include/c++/4.7/mutex:163:5: error: declared here 
In file included from /usr/include/c++/4.7/utility:72:0, 
       from /usr/include/c++/4.7/unordered_map:38, 
       from f.cpp:1: 
/usr/include/c++/4.7/bits/stl_pair.h: In instantiation of ‘constexpr std::pair<typename std::__decay_and_strip<_T1>::__type, typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = std::mutex; typename std::__decay_and_strip<_T2>::__type = std::mutex; typename std::__decay_and_strip<_T1>::__type = int]’: 
/usr/include/c++/4.7/bits/hashtable_policy.h:463:24: required from ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](_Key&&) [with _Key = int; _Pair = std::pair<const int, std::mutex>; _Hashtable = std::_Hashtable<int, std::pair<const int, std::mutex>, std::allocator<std::pair<const int, std::mutex> >, std::_Select1st<std::pair<const int, std::mutex> >, std::equal_to<int>, std::hash<int>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>; std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = std::mutex]’ 
f.cpp:6:8: required from here 
/usr/include/c++/4.7/bits/stl_pair.h:273:72: error: use of deleted function ‘constexpr std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = std::mutex; std::pair<_T1, _T2> = std::pair<int, std::mutex>]’ 
/usr/include/c++/4.7/bits/stl_pair.h:120:17: note: ‘constexpr std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = std::mutex; std::pair<_T1, _T2> = std::pair<int, std::mutex>]’ is implicitly deleted because the default definition would be ill-formed: 
/usr/include/c++/4.7/bits/stl_pair.h:120:17: error: use of deleted function ‘std::mutex::mutex(const std::mutex&)’ 
In file included from f.cpp:2:0: 
/usr/include/c++/4.7/mutex:163:5: error: declared here 

Answer 1

兩個選項。

一個是，你嘗試的第三件事m[1]應該編譯。你遇到了什麼錯誤？

二是使用emplace與piecewise_construct構造：

m.emplace(std::piecewise_construct, std::make_tuple(1), std::tuple<>()); 

這應該工作，但我認爲這很可能是有標準庫在那裏它沒有。