C++：无法为 Vertex 对象创建哈希函数

Posted 2023-02-22

【中文标题】C++：无法为 Vertex 对象创建哈希函数

【英文标题】：C++: Failing to make a hash function for a Vertex object

【发布时间】：2017-06-07 04:57:11

【问题描述】：

所以我正在尝试将 Graph 类作为另一个项目的一部分。顶点存储在由 unordered_map 定义的邻接列表中。我正在尝试创建一个哈希函数以允许我的 Vertex 类存储在此地图中，但我不知道我做错了什么。我收到此错误：

no matching function for call to ‘std::hash<Vertex<int> >::hash(int)’

这是我的 graph.h 文件

#ifndef GRAPH_GRAPH_H_
#define GRAPH_GRAPH_H_

#include <vector>
#include <unordered_map>

template <typename T>
class Vertex 
    private:
        T value;
    public:
        Vertex(T value);
        T getValue() const;
        void setValue(T value);

        //Equality check. This also requires an equality check to exist for T. If it does not (e.g. custom class), make sure you impemented one
        bool operator==(const Vertex<T>& v) const
            return (this->value == v.value);
        
;

template <typename T>
Vertex<T>::Vertex(T value) 
    this->value = value;


template <typename T>
T Vertex<T>::getValue() const
    return this->value;


template <typename T>
void Vertex<T>::setValue(T value) 
    this->value = value;


//If you want to hash a vertex, T needs a way of being hashed
namespace std 
    template<typename T> struct hash<Vertex<T>> 
        size_t operator()(Vertex<T> const& v) const 
            return hash(v.getValue());
        
    ;



template <typename T>
class Graph 
private:
    //Now I know what you're thinking. Graph theory taught me that a graph needs a set of nodes, and an adjacency list.
    //All you've given us is an adjacency list.
    //Well, since we need to use our nodes as keys, the set of keys is equivalent to the set of nodes. So there.

    unordered_map<Vertex<T>, std::vector<Vertex<T>>> adjacencyList;

public:
    Graph(); //Needs a hash function. For example, if your vertices are storing std::pairs, we need a hash to convert the pair into a key
             //Since I don't know what you will be storing, you should provide a hash function
    ~Graph();
    //Since we are initialising a new object, v must be whatever we are using to initialise our vertices
    void addVertex(T value, std::vector<T> adjacent);
    void removeVertex(T value);
    void printGraph();
;

template <typename T>
Graph<T>::Graph() 


template <typename T>
Graph<T>::~Graph() 


template <typename T>
void Graph<T>::addVertex(T value, std::vector<T> adjacent) 
    //Check that the value does not already exist
    if (this->adjacencyList.find(Vertex<T>(value)) == this->adjacencyList.end()) 
        Vertex<T> v = Vertex<T>(value);
        std::vector<Vertex<T>> adj;
        for (unsigned int i = 0; i < adjacent.size(); i++) 
            Vertex<T> vAdj = Vertex<T>(adjacent[i]);
            adj.push_back(vAdj);
        
        this->adjacencyList.insert(make_pair(v, adj));
    
        //If the values does not exist, place it and the adjacency in there
        //Otherwise, do nothing




template <typename T>
void Graph<T>::removeVertex(T value) 
    //Check that the value does not already exist
    //if (this->adjacencyList.find(Vertex<T>(value)) != this->adjacencyList.end()) 
        this->adjacencyList.erase(Vertex<T>(value));
    //


template <typename T>
void Graph<T>::printGraph() 


#endif /* GRAPH_GRAPH_H_ */

【参考方案1】：

你有一个错误

return hash(v.getValue());

std::hash 不是函数，是结构体，所以需要先创建一个对象。例如

return hash<T>(v.getValue()); 

【参考方案2】：

Vertex::getValue() 不能在hash<Vertex<T>> 内部调用，因为它没有标记为const。

您需要将其在类中的定义更改为

template <typename T>
class Vertex 
    ...
    T getValue() const;
    ...
;

同时，您也应该对operator== 应用相同的更改。

【讨论】：

所以我进行了更改，现在我收到消息 no matching function for call to 'std::hash >::hash(int)'