segfault with self-declared hash function-CodePudding

ChunkCorner.h

#pragma once
#include <filesystem>

#include <iostream>

class ChunkCorner
{
public:
    int x;
    int y;
    std::filesystem::path to_filename();
};

size_t hf(const ChunkCorner &chunk_corner);

bool eq(const ChunkCorner &c1, const ChunkCorner &c2);

ChunkCorner.cpp: fwiw: The hf and eq function implementation is based on the C Book p. 917.

#include "ChunkCorner.h"

using namespace std;

size_t hf(const ChunkCorner &chunk_corner)
{
    return hash<int>()(chunk_corner.x) ^ hash<int>()(chunk_corner.y);
}

bool eq(const ChunkCorner &c1, const ChunkCorner &c2)
{
    return c1.x == c2.x && c1.y == c2.y;
}

[...]

In another bit of code I use the class as follows:

unordered_set<ChunkCorner, decltype(&hf), decltype(&eq)> chunks_to_load {};

ChunkCorner c {1,2};
chunks_to_load.insert(c);

On the insert call I get a segfault (determined using breakpoints).

I use VS Code and when I launch the program in debug mode, it jumps to the following bit on segfault in hashtable_policy.h:

      __hash_code
      _M_hash_code(const _Key& __k) const
      {
    static_assert(__is_invocable<const _H1&, const _Key&>{},
        "hash function must be invocable with an argument of key type");
    return _M_h1()(__k);
      }

I am new to C and have trouble understanding what the issue is and I am not sure how to proceed debugging...

CodePudding user response：

You need to pass the hash and equals functions to your constructor. You've declared their type in the type arguments, which is going to be a pointer to function, but you haven't passed them in. So they're likely being zero initalized, so nullptr. Using them correctly should be done like this:

unordered_set<ChunkCorner, decltype(&hf), decltype(&eq)> chunks_to_load {16, hf, eq};

However, what I recommend is rewrite your Hash/Equals functions into function objects. This way, the default operations will work properly.

struct MyHasher {
  size_t operator()(const ChunkCorner &chunk_corner) const
   {
     return hash<int>()(chunk_corner.x) ^ hash<int>()(chunk_corner.y);
   }
};

struct MyEq {
 bool operator()(const ChunkCorner &c1, const ChunkCorner &c2) const
  {
    return c1.x == c2.x && c1.y == c2.y;
  }
};