C++ unordered_set 向量

2021-12-21 00:00:00 vector c++ c++11

我可以在 C++ 中创建一个 unordered_set 向量吗?像这样

Can I create a unordered_set of vectors in C++? something like this

std::unordered_set<std::vector<int>> s1;

因为我知道使用 std lib 的set"类是可能的，但似乎它不适用于无序版本谢谢

because I know that is possible with the "set" class of the std lib but seems that it doesn't work for the unordered version thanks

更新:这正是我要使用的代码

Update: this is the exactly code that I'm trying to use

typedef int CustomerId;
typedef std::vector<CustomerId> Route;
typedef std::unordered_set<Route> Plan;

// ... in the main
Route r1 = { 4, 5, 2, 10 };
Route r2 = { 1, 3, 8 , 6 };
Route r3 = { 9, 7 };
Plan p = { r1, r2 };

如果我使用set就可以了，但是在尝试使用无序版本时我收到一个编译错误

and it's all right if I use set, but I receive a compilation error when try to use the unordered version

main.cpp:46:11: error: non-aggregate type 'Route' (aka 'vector<CustomerId>') cannot be initialized with an initializer list
    Route r3 = { 9, 7 };

推荐答案

当然可以.不过，您必须提出一个哈希值，因为默认值 (std::hash>) 将不会实现.例如，基于这个答案，我们可以构建:

Sure you can. You'll have to come up with a hash though, since the default one (std::hash<std::vector<int>>) will not be implemented. For example, based on this answer, we can build:

struct VectorHash {
    size_t operator()(const std::vector<int>& v) const {
        std::hash<int> hasher;
        size_t seed = 0;
        for (int i : v) {
            seed ^= hasher(i) + 0x9e3779b9 + (seed<<6) + (seed>>2);
        }
        return seed;
    }
};

然后:

using MySet = std::unordered_set<std::vector<int>, VectorHash>;

如果您愿意，您也可以为这种类型的 std::hash 添加专门化(注意这可以em> 是 std::vector 的未定义行为，但对于用户定义的类型绝对没问题):

You could also, if you so choose, instead add a specialization to std::hash<T> for this type (note this could be undefined behavior with std::vector<int>, but is definitely okay with a user-defined type):

namespace std {
    template <>
    struct hash<std::vector<int>> {
        size_t operator()(const vector<int>& v) const {
            // same thing
        }
    };
}

using MySet = std::unordered_set<std::vector<int>>;

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