map/unordered_map 具有不可移动、默认可构造值类型

Question

更新：它出现在23.5.4.3这里版本m[1]应该是有效的

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更新2：m[1]正在使用 gcc4.9.1

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

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我似乎无法在不触发已删除的复制构造函数（无论是实习生还是实习生）的某个级别上发生错误的情况下添加 amutex到mapmutexstd::pair持有互斥锁的实习生。

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

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全部编译失败。我真正想要的只是mutex在内部就地构建map.

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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。

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补充：一个完整​​的例子operator[]添加：在 gcc 和 clang 下对我来说失败的

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#include <unordered_map>\n#include <mutex>\n\nint main() {\n    std::unordered_map<int, std::mutex> m;\n    m[1];\n\n    return 0;\n}\n

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clang3.5 中错误最相关的部分

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/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\'\n        : first(std::forward<_U1>(__x)), second(__y) { }\n                                         ^      ~~~\n...\n\nf.cpp:6:6: note: in instantiation of member function \'std::__detail::_Map_base<...>::operator[]\' requested here\n    m[1];\n     ^\n/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\n    mutex(const mutex&) = delete;\n    ^\n

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和从海湾合作委员会4.7.2

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In file included from /usr/include/c++/4.7/utility:72:0,\n                 from /usr/include/c++/4.7/unordered_map:38,\n                 from f.cpp:1:\n/usr/include/c++/4.7/bits/stl_pair.h: In instantiation of \xe2\x80\x98constexpr std::pair<_T1, _T2>::pair(_U1&&, const _T2&) [with _U1 = int; <template-parameter-2-2> = void; _T1 = int; _T2 = std::mutex]\xe2\x80\x99:\n/usr/include/c++/4.7/bits/stl_pair.h:273:72:   required from \xe2\x80\x98constexpr 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]\xe2\x80\x99\n/usr/include/c++/4.7/bits/hashtable_policy.h:463:24:   required from \xe2\x80\x98std::__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]\xe2\x80\x99\nf.cpp:6:8:   required from here\n/usr/include/c++/4.7/bits/stl_pair.h:126:45: error: use of deleted function \xe2\x80\x98std::mutex::mutex(const std::mutex&)\xe2\x80\x99\nIn file included from f.cpp:2:0:\n/usr/include/c++/4.7/mutex:163:5: error: declared here\nIn file included from /usr/include/c++/4.7/utility:72:0,\n                 from /usr/include/c++/4.7/unordered_map:38,\n                 from f.cpp:1:\n/usr/include/c++/4.7/bits/stl_pair.h: In instantiation of \xe2\x80\x98constexpr 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]\xe2\x80\x99:\n/usr/include/c++/4.7/bits/hashtable_policy.h:463:24:   required from \xe2\x80\x98std::__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]\xe2\x80\x99\nf.cpp:6:8:   required from here\n/usr/include/c++/4.7/bits/stl_pair.h:273:72: error: use of deleted function \xe2\x80\x98constexpr std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = std::mutex; std::pair<_T1, _T2> = std::pair<int, std::mutex>]\xe2\x80\x99\n/usr/include/c++/4.7/bits/stl_pair.h:120:17: note: \xe2\x80\x98constexpr std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = std::mutex; std::pair<_T1, _T2> = std::pair<int, std::mutex>]\xe2\x80\x99 is implicitly deleted because the default definition would be ill-formed:\n/usr/include/c++/4.7/bits/stl_pair.h:120:17: error: use of deleted function \xe2\x80\x98std::mutex::mutex(const std::mutex&)\xe2\x80\x99\nIn file included from f.cpp:2:0:\n/usr/include/c++/4.7/mutex:163:5: error: declared here\n

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Answer 1

两个选择。

一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emplace与piecewise_construct构造函数一起使用：

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

这应该可以工作，尽管我认为很可能存在标准库而没有。