我可以使用成员变量作为 hash_set/hash_map 的键吗？
我有一个这样的类：
class Foo
{
long long Id;
string x;
string y;
// other member variables and functions
};
我想将其存储在 hash_set （或 hash_map）中，但使用 Id 成员变量作为插入和搜索的键。我不知道我该怎么做。我想到了以下方法，但没有一个是真正好的：
1）我可以编写一个自定义哈希函数，该函数将使用 Id 对对象进行哈希处理，但随后我无法使用 find()hash_set 上的方法通过 Id (long long) 查找项目，因为它需要传入 Foo 对象。
2) 我可以复制 Id 并创建一个 hash_map 而不是 hash_set 但我有 1 亿个这些对象的实例所以我不想重复 Id 字段。
3）我可以将 Id 字段移到 Foo 之外，然后执行 hash_map，但这会有点混乱，因为 Id 在内部使用类，最好将其保留在 Foo 中。
有什么想法吗？我正在寻找的是一种存储 Foo 对象的方法，但能够使用 long long 在 hash_set 中搜索它们（通过ID）。
谢谢！
I have a class like this:
class Foo
{
long long Id;
string x;
string y;
// other member variables and functions
};
I would like to store this in a hash_set (or hash_map), but use the Id member variable as the key for inserting and searching. I'm not sure how I can do this. I thought of the following ways, but none of them are really good:
1) I can write a custom hash function that will hash the object using the Id, but then I can't use the find() method on hash_set to lookup the item by Id (long long) since it will require a Foo object to be passed-in.
2) I can duplicate the Id and create ahash_map<long long, Foo> instead of a hash_set<long long, Foo> but I have 100 million instances of these objects so I'd rather not duplicate the Id field.
3) I can move the Id field outside of Foo and then do hash_map<long long, Foo>, but it would be kind of messy since the Id is used internally by the class and it would be better to keep it with Foo.
Any ideas? What I'm looking for is a way to store Foo objects but be able to search for them in the hash_set using a long long (by Id).
Thanks!
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这是标准地图和集合容器的常见问题。添加一个构造函数或静态成员来创建一个比较对象，一个只有键成员有效的对象。
This is a common problem with the standard map and set containers. Add a constructor or static member that creates a comparison object, an object with only the key member valid.
选项 2 是您给出的三个选项中最好的选择。如果您愿意，您可以编写一个自定义 hash_set （甚至只是常规 hash_set 的包装器）来提供您想要的内容。在这种情况下，我更喜欢选项 1。您可以轻松添加一个仅接受键值的查找函数，并对其进行调整以正确使用内部查找函数，从而为您带来所有好处。
Option 2 is your best bet among the three you gave. If you're up to it, you could write a custom hash_set (even just a wrapper around the regular one) to provide what you want. In that case, I'd prefer option 1. You could easily add a find function that takes just the key value and adapts it to use the internal find function properly giving you all benefits.
我对此没有太多经验，但 boost::multiindex 默认构建在 boost::hash 之上，并且明确支持仅使用对象中找到的键在哈希中查找对象。
I don't have much experience with it, but boost::multiindex is built on top of boost::hash by default, and has explicit support for looking up an object in a hash using only a key found in the object.
如果您没有找到 hash_set/hash_map 容器的实际解决方案，您可能需要考虑使用 uthash哈希表实现，直接支持您的用例。虽然它是 C 语言，并且基于宏，但它是现有的最流行的哈希表实现之一。
If you don't find an actual solution for the hash_set/hash_map containers, you may want to consider using the uthash hash table implementation, which supports your use case directly. It's C though and it's based on macros but it's one of the most popular hash table implementations that exist.
这可能不是一个特别优雅的解决方案，但它有效：为排序对象的类定义一个
operator <（以及operator ==，正如 CashCow 建议的那样）基于Id字段，然后当您想要进行查找时，传入一个包含您要查找的Id的虚拟对象。This may not be a particularly elegant solution, but it works: define an
operator <(andoperator ==as well, as CashCow suggested) for your class that orders objects based on theIdfield, then when you want to do afind, pass in a dummy object that contains theIdyou're looking for.