std :: unordered_map :: find是否使用与Key类型不同的类型？ [英] std::unordered_map::find using a type different than the Key type?

问题描述

我有一个 unordered_map ，它使用字符串类型作为键：

I have an unordered_map that uses a string-type as a key:

std::unordered_map<string, value> map;

提供了 std :: hash 专业化 string ，以及合适的
operator == 。

A std::hash specialization is provided for string, as well as a suitable operator==.

现在我还有一个字符串视图类，它是指向现有字符串的弱指针，避免了堆分配：

Now I also have a "string view" class, which is a weak pointer into an existing string, avoiding heap allocations:

class string_view {
    string *data;
    size_t begin, len;
    // ...
};  

现在，我希望能够使用<$检查地图中是否存在键c $ c> string_view 对象。不幸的是， std :: unordered_map :: find 接受 Key 参数，而不是通用的 T 参数。

Now I'd like to be able to check if a key exists in the map using a string_view object. Unfortunately, std::unordered_map::find takes a Key argument, not a generic T argument.

（当然，我可以将一个提升为字符串，

(Sure, I can "promote" one to a string, but that causes an allocation I'd like to avoid.)

我想要的是类似

template<class Key, class Value>
class unordered_map
{
    template<class T> iterator find(const T &t);
};

这需要 operator ==（T，Key）和 std :: hash< T>（）进行适当定义，并将迭代器返回到匹配值。

which would require operator==(T, Key) and std::hash<T>() to be suitably defined, and would return an iterator to a matching value.

有什么解决方法吗？

推荐答案

P0919R2对无序容器的异构查找已在C语言中合并++ 2a的工作草案！

P0919R2 Heterogeneous lookup for unordered containers has been merged in the C++2a's working draft!

摘要似乎很合适我原来的问题：-）

The abstract seems a perfect match w.r.t. my original question :-)

摘要

该提案增加了异构查询支持到C ++标准库中的无序关联容器。结果，当提供不同（但兼容）的类型作为成员函数的键时，不需要创建临时键对象。

Abstract

This proposal adds heterogeneous lookup support to the unordered associative containers in the C++ Standard Library. As a result, a creation of a temporary key object is not needed when different (but compatible) type is provided as a key to the member function. This also makes unordered and regular associative container interfaces and functionality more compatible with each other.

通过本文提出的更改，以下代码将可以正常工作，而不会影响性能：

With the changes proposed by this paper the following code will work without any additional performance hits:

template<typename Key, typename Value>
using h_str_umap = std::unordered_map<Key, Value, string_hash>;
h_str_umap<std::string, int> map = /* ... */;
map.find("This does not create a temporary std::string object :-)"sv);

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