如何将unordered_set与自定义结构一起使用？ [英] How can I use an unordered_set with a custom struct?

问题描述

我想使用 unordered_set 和自定义 struct 。在我的情况下，自定义 struct 表示欧氏平面中的2D点。我知道应该定义一个散列函数和比较器运算符，并且已经做到了，正如您在下面的代码中看到的那样：

I want to use an unordered_set with a custom struct. In my case, the custom struct represents a 2D point in an euclidean plane. I know that one should define a hash function and comparator operator and I have done so as you can see in my code below:

struct Point {
    int X;
    int Y;

    Point() : X(0), Y(0) {};
    Point(const int& x, const int& y) : X(x), Y(y) {};
    Point(const IPoint& other){
        X = other.X;
        Y = other.Y;
    };

    Point& operator=(const Point& other) {
        X = other.X;
        Y = other.Y;
        return *this;
    };

    bool operator==(const Point& other) {
        if (X == other.X && Y == other.Y)
            return true;
        return false;
    };

    bool operator<(const Point& other) {
        if (X < other.X )
            return true;
        else if (X == other.X && Y == other.Y)
            return true;

        return false;
    };

    size_t operator()(const Point& pointToHash) const {
        size_t hash = pointToHash.X + 10 * pointToHash.Y;
        return hash;
    };
};

但是，如果我将集合定义如下，则会出现以下错误：

However, I'm getting the error below, if I define the set as follows:

unordered_set<Point> mySet;

错误C2280'std :: hash< __ Kty> :: hash（ const std :: hash< _Kty>&）'：
尝试引用已删除的函数

Error C2280 'std::hash<_Kty>::hash(const std::hash<_Kty> &)': attempting to reference a deleted function

我缺少什么？

推荐答案

std :: unordered_set的第二个模板参数是用于哈希的类型。并且在您的情况下默认为 std :: hash< Point> （不存在）。因此，如果散列器类型相同，则可以使用 std :: unordered_set< Point，Point> 。

The second template parameter to std::unordered_set is the type to use for hashing. and will default to std::hash<Point> in your case, which doesn't exist. So you can use std::unordered_set<Point,Point> if the hasher is the same type.

如果不想指定散列器，则为 Point 定义 std :: hash 的特化，然后除掉成员函数，并在您的专长的 operator（）主体中实现散列，或从std :: hash专长调用该成员函数。

Alternatively if you do not want to specify the hasher, define a specialization of std::hash for Point and either get rid of the member function and implement the hashing in the body of your specialization's operator(), or call the member function from the std::hash specialization.

#include <unordered_set>

struct Point {
    int X;
    int Y;

    Point() : X(0), Y(0) {};
    Point(const int& x, const int& y) : X(x), Y(y) {};
    Point(const Point& other){
        X = other.X;
        Y = other.Y;
    };

    Point& operator=(const Point& other) {
        X = other.X;
        Y = other.Y;
        return *this;
    };

    bool operator==(const Point& other) const {
        if (X == other.X && Y == other.Y)
            return true;
        return false;
    };

    bool operator<(const Point& other) {
        if (X < other.X )
            return true;
        else if (X == other.X && Y == other.Y)
            return true;

        return false;
    };

    // this could be moved in to std::hash<Point>::operator()
    size_t operator()(const Point& pointToHash) const noexcept {
        size_t hash = pointToHash.X + 10 * pointToHash.Y;
        return hash;
    };

};

namespace std {
    template<> struct hash<Point>
    {
        std::size_t operator()(const Point& p) const noexcept
        {
            return p(p);
        }
    };
}


int main()
{
    // no need to specify the hasher if std::hash<Point> exists
    std::unordered_set<Point> p;
    return 0;
}

演示

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