如何使用 unordered

本文介绍了如何使用 unordered_set?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

I am trying to define an unordered_set like this:

unordered_set<Point> m_Points;

When I compile it, I get the following error:

The C++ Standard doesn't provide a hash for this type.

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); } };

• How/where do I define a hash function for Point?
• What would be a good hash function for a 2D point?

解决方案

std::unordered_set requires you to write hash functions to store and find your own types.

Base types and many types in the std namespace do have such hash functions within std::hash<Key>. These functions follow certain rules:

Accepts a single parameter of type Key.

Returns a value of type size_t that represents the hash value of the parameter.

Does not throw exceptions when called.

For two parameters k1 and k2 that are equal, std::hash<Key>()(k1) == std::hash<Key>()(k2).

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().

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.

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()) ); } }; }

And we are done.