如何在 unordered_map 中使用向量变体作为键？

Question

如何使用变体向量作为 unordered_map 中的键？

例如，我想让下面的代码工作。

using VariantType = std::variant<int, std::string, unsigned int>;
using VecVariantType = std::vector<VariantType>;
std::unordered_map<VecVariantType, int, $some_hash_function$> m;

如何实施 $some_hash_function$？

Answer 1

我喜欢一个好主意...

除了“我想知道我是否可以编译它和运行”示例之外，我不知道您如何或为什么将这样的东西用于其他东西。

#include <iostream>
#include <vector>
#include <unordered_map>
#include <variant>

using VariantType = std::variant<int, std::string, unsigned int>;
using VecVariantType = std::vector<VariantType>;

struct Hasher
{
    size_t operator()(const VecVariantType &v) const
    {
        size_t hash = 0;
        for (auto &val : v)
        {
            hash ^= std::hash<VariantType>()(val);
        }
        return hash;
    }
};

std::ostream& operator<<(std::ostream& out, std::pair<const VecVariantType, int> &p)
{
    out << "key: { ";
    bool needs_comma = false;
    for (auto &var : p.first)
    {
        if (needs_comma)
        {
            out << ", ";
        }
        if (std::holds_alternative<int>(var))
        {
            out << "int: " << std::get<int>(var);
            needs_comma = true;
        }
        if (std::holds_alternative<std::string>(var))
        {
            out << "string: " << std::get<std::string>(var);
            needs_comma = true;
        }
        if (std::holds_alternative<unsigned int>(var))
        {
            out << "uint: " << std::get<unsigned int>(var);
            needs_comma = true;
        }
    }
    out << " }, value: " << p.second;
    return out;
}

void lookup(const VecVariantType &var, std::unordered_map<VecVariantType, int, Hasher> &m)
{
    std::cout << "Lookup ";
    auto it = m.find(var);
    if (it != m.end())
    {
        std::cout << "success - " << *it << "\n";
    }
    else
    {
        std::cout << "failure\n";
    }
}

int main()
{
    std::unordered_map<VecVariantType, int, Hasher> m;
    auto one = VecVariantType { 1, "one", 1u };
    auto two = VecVariantType { 2, "two", 2u };
    auto three = VecVariantType { 3, "three", 3u };
    auto nnn = VecVariantType { 1, "one", 1u, 2, "two", 2u, 3, "three", 3u };
    m.emplace(one, 1);
    m.emplace(two, 2);
    m.emplace(three, 3);
    m.emplace(nnn, 999);

    std::cout << "Enumerating:\n";
    for (auto& item : m)
    {
        std::cout << "    " << item << "\n";
    }

    lookup(one, m);
    lookup(two, m);
    lookup(three, m);
    lookup(nnn, m);
}

https://repl.it/repls/AmazingCrushingOpen64

如何在 unordered_map 中使用向量变体作为键？

How to use a vector variants as key in unordered_map?

c++

hash

unordered-map

variant