map/unordered_map 具有不可移动的默认可构造值类型

Question

更新：在 23.5.4.3 here 中，m[1] 版本应该是有效的

Update2：m[1] 正在使用 gcc4.9.1

具体来说，std::mutex。假设我想要一个std::unordered_map<int, std::mutex>。这可能吗？

如果mutex 或持有互斥锁的实习生std::pair 在某个级别上触发已删除的复制构造函数上的错误，我似乎无法将mutex 添加到map。

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

全部编译失败。我真正想要的只是在map 内就地构建mutex。

我宁愿不使用std::unique_ptr<std::mutex>，但我知道这可能是我最好的选择。

添加：operator[] 的完整示例在 gcc 和 clang 下对我来说失败

#include <unordered_map>
#include <mutex>

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

    return 0;
}

clang3.5 中最相关的错误部分

/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;
    ^

来自 gcc 4.7.2

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

两个选项。

一个是你尝试的第三件事，m[1]，应该编译。你遇到了什么错误？

第二种是将emplace与piecewise_construct构造函数一起使用：

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

这个应该工作，虽然我认为很可能有一些标准库在那里不工作。