Class 与相同 class 的指针的 std::unordered_set

Question

I am trying to create a class that has contains an std::unordered_set of pointers to the same class but am unable to find a way to prepare the hash function before declaring the class.

struct hash
{
   inline std::size_t operator()(Vertex const * & other);
};

struct Vertex
{
    // ...
    double x, y, z;
    std::unordered_set<Vertex *, hash> touching;
};

inline std::size_t hash::operator()(Vertex * const & other) const
{
    return ((hash<double>()(other->x))>>1)^
            ((hash<double>()(other->y))   )^
            ((hash<double>()(other->z))<<1);
}

Answer 1

我假设您的意思是std::hash ，在hash::operator() ，如果是这样指定完整的 scope 并包括<functional> 。 那么都需要一个Vertex class的前向声明，那么一切都好

#include <unordered_set>
#include <functional>  // std::hash

struct Vertex;  // forward declare the `Vertex` 

struct hash
{
   std::size_t operator()(Vertex* other) const;
};

struct Vertex
{
   double x, y, z;
   std::unordered_set<Vertex*, ::hash> touching;
};

std::size_t hash::operator()(Vertex* other) const
{
   return ((std::hash<double>()(other->x)) >> 1) ^
      ((std::hash<double>()(other->y))) ^
      ((std::hash<double>()(other->z)) << 1);
}

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另外，请注意，您不需要获取指针的const -ref（即Vertex const*& other ），只需按原始类型的值传递即可。