无序 - 多对地图 [英] Unordered-MultiMap of Pairs

问题描述

我很难成对有效地存储信息：

例如，我有两个结构体表示（x，y）坐标，我希望计算和存储距离之间。目前我在一个

unordered_map< pair< struct1 *，Struct2 *，double>

我的问题是，在搜索时我想要结果< Struct1 *，Struct2 *> 与< Struct2 *，Struct1 *> 相同的值，那样我不必存储信息两次。我想过使用多图，但我认为 std :: hash< pair< pointer1，pointer2>> 将哈希与 pair< pointer2，pointer1> 有关如何执行此操作的任何建议？

编辑：

$ b b

我想过做一个客户散列，只是使用std :: hash来添加指针的两个散列值：

size_t operator（）（const pair< Location *，Location *>& key）
{
hash< Location * hash1;
return（hash1（key.first）+ hash1（key.second））;
}

当我调用find（struct1，struct2），但是不是当我调用find（struct2，struct1）

解决方案

unordered_map不仅使用散列来标识密钥，还使用比较运算符：

 模板< 
 class Key，
 class T，
 class Hash = std :: hash< Key>，
 class KeyEqual = std :: equal_to< Key> 
 class Allocator = std :: allocator< std :: pair< const Key，T> > 
> class unordered_map; 
  

它是 KeyEqual 调用 std :: equal_to 其中默认调用 operator == 。

要做你想要的，你可以替换 KeyEqual 参数，当给定一个对< struct1 *，Struct2 * < / c $ c>将返回 true 独立于订单。

I'm struggling to efficiently store information in pairs:

For instance I have two structs that represent (x,y) coords that i wish to calculate and store the distance between. Currently I am store all the values twice in a

unordered_map<pair<Struct1*,Struct2*,double>

My problem is that when searching I want the result <Struct1*,Struct2*> to turn up the same value as <Struct2*,Struct1*> that way I do not have to store information twice. I've thought about using a multimap but I think that std::hash<pair<pointer1,pointer2>> will hash to the same value as pair<pointer2,pointer1> any suggestions on how to do this?

Edit:

I've thought about doing a customer hash that simply adds the two hash values of the pointers using std::hash like:

size_t operator() (const pair<Location*,Location*> &key) { hash<Location*> hash1; return (hash1(key.first) + hash1(key.second)); }

this works when I call find(struct1,struct2), however not when i call find(struct2,struct1)

解决方案

unordered_map does not only use the hash to identify a key, it also uses a comparison operator :

template<
    class Key,
    class T,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator< std::pair<const Key, T> >
> class unordered_map;

It is the KeyEqual parameter which by default calls std::equal_to which by default calls operator==.

To do what you want, you can replace the KeyEqual parameter whith a custom one that when given a pair<struct1*, Struct2*> and a pair<struct2*, struct 1*> will return true independently of the order.

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