C ++ std :: map命名需求分配器感知容器 [英] C++ std::map named requirements allocator-aware container

问题描述

当前，我正在尝试将一些C ++的命名需求转换为C ++ 20概念( https://godbolt.org/z/EdY5d6319 仍然是WIP！).但是，我遇到了一个问题.

Currently I'm trying to convert some of C++'s named requirements to C++20 concepts (https://godbolt.org/z/EdY5d6319 still highly WIP!). However, I stumpled upon a problem.

给出最终的C ++ 20工作草案(http://open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf ):

Given the final C++20 working draft (http://open-std.org/jtc1/sc22/wg21/docs/papers/2020/n4861.pdf):

• 第790页:对可识别分配器的容器的要求:
  给定具有value_type T的分配器感知容器X，表达式 a == t 其中 a 表示类型X和 t 的非常量左值类型X的左值或常量值是必需的.
  前提条件:T是Cpp17CopyInsertable，可以插入X和 Cpp17CopyAssignable 中.
  ->因此， X :: value_type 必须是CopyAssignable.
• 第699页: is_copy_assignable :
  对于可引用的类型T，结果与 is_assignable_v< T& ;，常量T& ，否则为 false ."
• 第849页: std :: map :
  地图满足容器，可逆容器(22.2)，关联容器的所有要求.容器(22.2.6)和可识别分配器的容器(表76)."

• page 790: requirements for an allocator-aware container:
  Given an allocator-aware container X with value_type T the expression a == t where a denotes a non-const lvalue of type X and t an lvalue or const rvalue of type X is required.
  Preconditions: T is Cpp17CopyInsertable into X and Cpp17CopyAssignable.
  -> Therefore, X::value_type must be CopyAssignable.
• page 699: is_copy_assignable:
  "For a referenceable type T, the same result as is_assignable_v<T&, const T&>, otherwise false."
• page 849: std::map:
  "A map meets all of the requirements of a container, of a reversible container (22.2), of an associative container (22.2.6), and of an allocator-aware container (Table 76)."

因此 std :: map< Key，T> 满足可识别分配器的容器的要求.因此， std :: map< Key，T> :: value_type 必须是CopyAssignable.但是， std :: map< Key，T> :: value_type 定义为 std :: pair< const Key，T> ，不是 Key 成为 const ，因此strong> CopyAssignable.因此， std :: map 不满足可识别分配器的容器要求，这与标准草稿中的第849页相矛盾.

So std::map<Key, T> meets the requirements of an allocator-aware container. Therefore, std::map<Key, T>::value_type must be CopyAssignable. However, std::map<Key, T>::value_type is defined as std::pair<const Key, T>, which is not CopyAssignable due to Key beeing const. Therefore, std::map does not meet the allocator-aware container requirements, which contradicts page 849 in the standard draft.

我在这里想念什么?

推荐答案

在

除 map 和 multimap 之外，关联容器满足分配器感知容器的所有要求，而这些要求放在 value_type 中表76 适用于 key_type 和 mapped_type .[注意:例如，在某些情况下，即使关联的 value_type ， pair< const key_type，mapped_type> 不是 Cpp17CopyAssignable .—尾注]

The associative containers meet all the requirements of Allocator-aware containers, except that for map and multimap, the requirements placed on value_­type in Table 76 apply instead to key_­type and mapped_­type. [ Note: For example, in some cases key_­type and mapped_­type are required to be Cpp17CopyAssignable even though the associated value_­type, pair<const key_­type, mapped_­type>, is not Cpp17CopyAssignable. — end note ]

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