将boost:：hash与boost:：property_树一起使用？

我试图为包含boost:：property_树（

boost:：property_树：：basic_树）的对象生成哈希值。在boost头文件中搜索属性树时，我找不到任何定义的哈希值函数。我正在努力实现的基本示例：

class MyClass{
public:
    friend std::size_t hash_value(const MyClass & obj);
private:
    boost::property_tree m_data;
}

inline std::size_t hash_value(const MyClass & obj){
    std::size_t seed = 0;
    boost::hash_combine(seed,obj.m_data);
    return seed;
}

此代码将无法编译，原因是：
“没有匹配的函数用于调用'hash_value（const boost:：property_tree&'）”

我的问题是：在我没有找到的某个头文件中是否为boost:：property_树定义了hash_值。如果不是，那么通过遍历属性树散列属性树的boost惯用方法是什么

我应该使用ptree序列化来转换为std:：string和hash，还是手动遍历树并创建递归hash？
只需为ptree专门化
hash
：

#include <boost/property_tree/ptree.hpp>
#include <boost/functional/hash.hpp>

namespace boost {
    template<typename Key, typename Data, typename KeyCompare>
    struct hash<boost::property_tree::basic_ptree<Key, Data, KeyCompare> > {
        size_t operator()(boost::property_tree::basic_ptree<Key, Data, KeyCompare> const& pt) const {
            std::size_t seed = 0;
            boost::hash_combine(seed, pt.template get_value<std::string>());
            boost::hash_range(seed, pt.begin(), pt.end());
            return seed;
        }
    };
}

现在你可以测试它了


#include <iostream>

int main() {
    for (std::string const data : {
            R"({"a":[1,2,3],"b":{"nest":"hello","more":"world"}})",
            R"({"b":{"nest":"hello","more":"world"},"a":[1,2,3]})",
            R"({ })",
            R"({})",
        })
    {
        MyClass o(data);
        std::cout << "object hash: " << hash_value(o) << " " << data << "\n";
    }
}

注意：散列和相等
对于许多容器，hash假设一个对应的相等比较器。如果他们不匹配，你会被解雇

您可能会试图根据
ptree
的有序（关联）接口定义
hash\u范围：

boost::hash_range(seed, pt.ordered_begin(), pt.not_found()); // CAUTION

这样做的优点是
{a:1，“b:2}
将匹配
{b:2，“a:1}

不要这样做，除非你知道自己在做什么。具体来说，您需要向使用此哈希的每个容器/算法传递一个兼容的相等比较器

如果您这样编写，并使用如下驱动程序进行测试：

int main() {
    MyClass a{ R"({"a":[1,2,3],"b":{"nest":"hello","more":"world"}})" },
            b{R"({"b":{"nest":"hello","more":"world"},"a":[1,2,3]})" },
            c{R"({ })" },
            d{R"({})" };

    for (auto& lhs : {a,b,c,d})
    for (auto& rhs : {a,b,c,d})
    {
        std::cout << "hash: " << hash_value(lhs) << " " << hash_value(rhs) << " - equality: " << std::boolalpha << (lhs==rhs) << "\n";
        if ((hash_value(lhs) == hash_value(rhs)) != (lhs==rhs))
            std::cout << " -- MISMATCH\n";
    }
}

不匹配警告表示相等和哈希不一致

如果使用原始散列（折叠上方）运行测试驱动程序，它将打印：

hash: 10737438301360613971 10737438301360613971 - equality: true
hash: 10737438301360613971 10737438301360613971 - equality: false
 -- MISMATCH
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 10737438301360613971 - equality: false
 -- MISMATCH
hash: 10737438301360613971 10737438301360613971 - equality: true
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 3864292196 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true

hash: 3573231694259656572 3573231694259656572 - equality: true
hash: 3573231694259656572 11176663460548092204 - equality: false
hash: 3573231694259656572 3864292196 - equality: false
hash: 3573231694259656572 3864292196 - equality: false
hash: 11176663460548092204 3573231694259656572 - equality: false
hash: 11176663460548092204 11176663460548092204 - equality: true
hash: 11176663460548092204 3864292196 - equality: false
hash: 11176663460548092204 3864292196 - equality: false
hash: 3864292196 3573231694259656572 - equality: false
hash: 3864292196 11176663460548092204 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3573231694259656572 - equality: false
hash: 3864292196 11176663460548092204 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true

不要使用序列化，因为它容易更改表示/松散信息。刚刚意识到您只能使用ptree专门化（通过boost，而不是）。我根据您的答案使用了一个实现，然后在其中发现了一个bug。回到您的答案，我能够在您提供的示例中重现错误：。当前的实现没有检查ptree中项目的值，只检查键。@Carter12s您完全正确。令人尴尬的是，我忘记了节点可以有子节点以外的“内容”值。固定的。你是C++的上帝！我刚刚自己想出了解决办法，但你已经抢先一步了。谢谢你的帮助。有小费罐吗？
hash: 10737438301360613971 10737438301360613971 - equality: true
hash: 10737438301360613971 10737438301360613971 - equality: false
 -- MISMATCH
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 10737438301360613971 - equality: false
 -- MISMATCH
hash: 10737438301360613971 10737438301360613971 - equality: true
hash: 10737438301360613971 3864292196 - equality: false
hash: 10737438301360613971 3864292196 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 10737438301360613971 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true

hash: 3573231694259656572 3573231694259656572 - equality: true
hash: 3573231694259656572 11176663460548092204 - equality: false
hash: 3573231694259656572 3864292196 - equality: false
hash: 3573231694259656572 3864292196 - equality: false
hash: 11176663460548092204 3573231694259656572 - equality: false
hash: 11176663460548092204 11176663460548092204 - equality: true
hash: 11176663460548092204 3864292196 - equality: false
hash: 11176663460548092204 3864292196 - equality: false
hash: 3864292196 3573231694259656572 - equality: false
hash: 3864292196 11176663460548092204 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3573231694259656572 - equality: false
hash: 3864292196 11176663460548092204 - equality: false
hash: 3864292196 3864292196 - equality: true
hash: 3864292196 3864292196 - equality: true