我想这样定义一个unordered_set:
I am trying to define an unordered_set like this:
unordered_set<Point> m_Points;当我编译它,我得到以下错误:
When I compile it, I get the following error:
C ++标准不为此类型提供散列。
The C++ Standard doesn't provide a hash for this type.
code> Point :
Class Point:
class Point{ private: int x, y; public: Point(int a_x, int a_y) : x(a_x), y(a_y) {} ~Point(){} int getX()const { return x; } int getY()const { return y; } bool operator == (const Point& rhs) const{ return x == rhs.x && y == rhs.y; } bool operator != (const Point& rhs) const{ return !(*this == rhs); } };
- 如何/在何处为点定义散列函数?
- 对于2D点,什么是好的哈希函数? 推荐答案
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接受键的单个参数。
在调用时不会抛出异常
对于两个参数 k1 和 k2 等于 std :: hash< Key>()(k1)== std :: hash< Key>()(k2)。
对于两个不同的参数 k1 和 k2 ,则 std :: hash< Key>()(k1)== std :: hash< Key>()(k2)的概率应该非常小, code> 1.0 / std :: numeric_limits< size_t> :: max()。
For two different parameters k1 and k2 that are not equal, the probability that std::hash<Key>()(k1) == std::hash<Key>()(k2) should be very small, approaching 1.0/std::numeric_limits<size_t>::max().
$ b b
现在我们得到了定义,让我们考虑一下什么是你的点结构的好的哈希函数。有请求 std :: pair (非常类似于点结构)有一个散列函数,但不幸的是,没有使它成为C ++ 11标准。
Now that we got the definitions out of the way, let's think about what would be a good hash function for your point structure. There was a request that std::pair (which is very similar to a point structure) got a hash function, but, unfortunately, that did not make it into the C++11 standard.
但我们很幸运:SO是真棒,当然,你基本上可以找到答案。注意,你不必自己散列整数,因为 std :: hash 已经有一个专门化。因此,让我们深入一下哈希函数,根据此答案:
But we are lucky: SO is awesome and, of course, you can basically already find the answer. Note that you do not have to hash integers yourself, because std::hash has a specialization for that already. So let's dig into our hash function, according to this answer:
namespace std { template <> struct hash<Point> { size_t operator()(Point const & x) const noexcept { return ( (51 + std::hash<int>()(x.getX())) * 51 + std::hash<int>()(x.getY()) ); } }; }我们完成了。
std :: unordered_set 需要您写散列函数来存储和查找自己的类型。
std::unordered_set requires you to write hash functions to store and find your own types.
基本类型和许多类型在 std 命名空间在 std :: hash< Key> 中有这样的散列函数。这些函数遵循某些规则:
Base types and many types in the std namespace do have such hash functions within std::hash<Key>. These functions follow certain rules:
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