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std::hash <std::optional>

STD：：散列<std：：可选>

Defined in header ​​
template<class T> struct hash<std::optional<T>>;​(since C++17)

模板的专门化std::hash为std::optional类允许用户获取包含在optional物品。

专业化std::hash<optional<T>>已启用%28(参见std::hash%29std::hash<std::remove_const_t<T>>已启用，否则将禁用。

当启用时，对象o类型std::optional<T>包含一个值，std::hash<std::optional<T>>()(o)计算值与std::hash<std::remove_const_t<T>>()(*o)对于不包含值的可选选项，未指定散列。

此专门化的成员函数不一定为no，除非基础类型的散列可能抛出。

模板参数

T-the type of the value contained in optional object

例

二次

#include <optional>
#include <unordered_set>
#include <string>
#include <iostream>
using namespace std::literals;
int main()
{
    // hash<optional> makes it possible to use unordered_set
    std::unordered_set<std::optional<std::string>> s = {
            "abc"s, std::nullopt, "def"s
    };
 
    for(const auto& o : s)
        std::cout << o.value_or("(null)") << ' ';
}

二次

可能的产出：

二次

def abc (null)

二次

另见

hash (C++11)hash function object (class template)

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http：//en.cppreference.com/w/cpp/实用程序/可选/散列

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