boost::unordered_map 中以结构为键的 boost 变体

Question

我在 boost 变体中有两个结构，variant 是 boost::unordered_map 的关键。我正在寻找实现哈希和等于方法的解决方案，该方法将采用变体中的结构。 下面是一个实现的 RouteHasher 方法（它在运行时不起作用（相同的键参数不会从映射中获取值）。如何实现一个将结构的两个成员都用于哈希的哈希？

struct v4RouteKey_t
{
   uint8 ipv4[4];
   uint32 val;
}
struct v6RouteKey_t
{
   uint8 ipv6[16];
   uint32 val;
}
typedef boost::variant < v4RouteKey_t, v6RouteKey_t > RouteKey;
typedef boost::unordered_map < RouteKey, RouteValue_t > RouteMap;

struct RouteHasher : public boost::static_visitor<std::size_t>
{
    template<typename T>
    std::size_t operator()(const T& x) const { return boost::hash<T>()(x); }
    std::size_t operator()(const RouteKey& x) const { return 
    boost::apply_visitor(RouteHasher(), x); }
};

struct RouteEquals : public boost::static_visitor<bool>
{
    template<typename T>
    bool operator()(const T& lhs, const T& rhs) const { return lhs == rhs; }

    template<typename T1, typename T2>
    bool operator()(const T1& lhs, const T2& rhs) const { return false; }

    bool operator()(const RouteKey& lhs, const RouteKey& rhs) const
    { return boost::apply_visitor(RouteEquals(), lhs, rhs); }
};

Answer 1

您是否为 v4RouteKey_t 和 v6RouteKey_t 类型实现了哈希（boost::hash_value 或 std::hash<>）和相等性？

添加使它可以为我编译：

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#include <boost/unordered_map.hpp>
#include <boost/variant.hpp>
#include <boost/array.hpp>
#include <boost/tuple/tuple.hpp>
#include <cstdint>

struct v4RouteKey_t { 
    boost::array<uint8_t, 4> ipv4; uint32_t val; 
    bool operator==(v4RouteKey_t const& other) const {
        return ipv4 == other.ipv4 && val == other.val;
    };
};
struct v6RouteKey_t { 
    boost::array<uint8_t, 16> ipv6; uint32_t val; 
    bool operator==(v6RouteKey_t const& other) const {
        return ipv6 == other.ipv6 && val == other.val;
    };
};

size_t hash_value(v4RouteKey_t const& key) { return key.val; }
size_t hash_value(v6RouteKey_t const& key) { return key.val; }

struct RouteValue_t {};

typedef boost::variant<v4RouteKey_t, v6RouteKey_t> RouteKey;
typedef boost::unordered_map<RouteKey, RouteValue_t> RouteMap;

int main() {
    RouteMap map;
}

奖金

启用 c++17 的库/编译器可以在没有提升的情况下完成所有这些工作：

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#include <unordered_map>
#include <numeric>
#include <variant>
#include <array>
#include <tuple>

struct v4RouteKey_t { 
    std::array<uint8_t, 4> ipv4; uint32_t val; 

    bool operator==(v4RouteKey_t const& other) const { return std::tie(ipv4, val) == std::tie(other.ipv4, other.val); };
};
struct v6RouteKey_t { 
    std::array<uint8_t, 16> ipv6; uint32_t val; 
    bool operator==(v6RouteKey_t const& other) const { return std::tie(ipv6, val) == std::tie(other.ipv6, other.val); };
};

namespace std {

    template <> struct hash<v4RouteKey_t> {
        size_t operator()(v4RouteKey_t const& key) { 
            return std::accumulate(key.ipv4.begin(), key.ipv4.end(), size_t(key.val), [](size_t seed, uint8_t b) { return (seed*139) ^ b; });
        }
    };
    template <> struct hash<v6RouteKey_t> {
        size_t operator()(v6RouteKey_t const& key) { 
            return std::accumulate(key.ipv6.begin(), key.ipv6.end(), size_t(key.val), [](size_t seed, uint8_t b) { return (seed*139) ^ b; });
        }
    };
}

struct RouteValue_t {};

typedef std::variant<v4RouteKey_t, v6RouteKey_t> RouteKey;
typedef std::unordered_map<RouteKey, RouteValue_t> RouteMap;

int main() {
    RouteMap map;
}