试着学习boost :: intrusive Q5

时间：2015-03-23 02:02:19

标签： c++ c++11 boost intrusive-containers

我有以下程序。我用linux下的gcc-4.9.2构建它。我的问题是：

1）为什么哈希表似乎是第一次排序，但是在从值中删除项后会丢失排序？

2）我如何通过键自己走散列表并说出std :: cout每个散列到存储桶的项目，例如the #if 0 #endif部分中的代码？ 

#include <vector>
#include <iostream>
#include <vector>
#include <functional>

#include <boost/intrusive/unordered_set.hpp>

namespace bic = boost::intrusive;

std::hash<std::string> hash_fn;

struct MyClass : bic::unordered_set_base_hook<bic::link_mode<bic::auto_unlink>>
{
    std::string name;
    int anInt1;
    mutable bool bIsMarkedToDelete;

    MyClass(std::string name, int i) : name(name), anInt1(i), bIsMarkedToDelete(false) {}

    bool operator==(MyClass const& o) const
    {
        //return anInt1 == o.anInt1 && name == o.name;
        return name == o.name;
    }

    struct hasher
    {
        size_t operator()(MyClass const& o) const
        {
            return o.anInt1;
            //return hash_fn(o.name);
        }
    };
};

std::ostream& operator << (std::ostream& out, const MyClass& ac)
{
    std::cout << ac.name << " " << ac.anInt1;

    return out;
}

typedef bic::unordered_set<MyClass, bic::hash<MyClass::hasher>, bic::constant_time_size<false> > HashTable;

int main()
{
    std::vector<MyClass> values
    {
        MyClass { "John",     0 },
        MyClass { "Mike",     0 },
        MyClass { "Dagobart", 25 },
        MyClass { "John",     5 },
        MyClass { "Mike",     25 },
        MyClass { "Dagobart", 26 },
        MyClass { "John",     10 },
        MyClass { "Mike",     25 },
        MyClass { "Dagobart", 27 },
        MyClass { "John",     15 },
        MyClass { "Mike",     27 }
    };

    HashTable::bucket_type buckets[100];
    HashTable hashtable(values.begin(), values.end(), HashTable::bucket_traits(buckets, 100));

    std::cout << "\nContents of std::vector<MyClass> values\n";

    for(auto& e: values)
        std::cout << e << " ";

    std::cout << "\nContents of HashTable hashtable\n";

    for(auto& b : hashtable)
        std::cout << b << '\n';

#if 0 // This code won't compile since there is no operator [] for hashtable
    for(int bucket = 0; bucket < 27; bucket++)
    {
        auto hit(hashtable[bucket].rbegin());
        auto hite(hashtable[bucket].rend());

        while (hit != hite)
        {
            MyClass mc = *hit;

            std::cout << mc << " ";

            hit++;
        }

        std::cout << '\n';
    }
#endif // 0

    std::cout << '\n';
    std::cout << "values size first " << values.size() << '\n';
    std::cout << "hash size fist " << hashtable.size() << '\n';

    for(auto& e: values)
        e.bIsMarkedToDelete |= ("Mike" == e.name);

    std::cout << "removing all bIsMarkedToDelete";
    for(auto& e: values)
        if(e.bIsMarkedToDelete)
            std::cout << e << " ";

    std::cout << '\n';

    values.erase(
        std::remove_if(std::begin(values), std::end(values), std::mem_fn(&MyClass::bIsMarkedToDelete)),
                       std::end(values));

    std::cout << "values size now " << values.size() << '\n';
    std::cout << "hash size now " << hashtable.size() << '\n';

    std::cout << "Contents of value after removing elements " << '\n';
    for(auto& e: values)
        std::cout << e << " ";

    std::cout << "\nContents of HashTable hashtable after delete Mike\n";

    for(auto& b : hashtable)
        std::cout << b << '\n';

    std::cout << '\n';

    values.clear();

    std::cout << values.size() << '\n';
    std::cout << hashtable.size() << '\n';

    std::cout << "Done\n";

    int j;
    std::cin >> j;
}

1 个答案:

答案 0 :(得分：1)

您的哈希值和相等性不一致，因此违反了容器不变量：

bool operator==(MyClass const& o) const
{
    //return anInt1 == o.anInt1 && name == o.name;
    return name == o.name;
}

struct hasher
{
    size_t operator()(MyClass const& o) const
    {
        return o.anInt1;
        //return hash_fn(o.name);
    }
};

这将是很好的IFF name的每个不同值总是散列到同一个桶。唉它没有：例如“迈克”哈希到3个不同的值：

    MyClass { "Mike",     0  },
    MyClass { "Mike",     25 },
    MyClass { "Mike",     25 },
    MyClass { "Mike",     27 }
  

1）为什么哈希表似乎是第一次排序，但是在从值中删除项后会丢失排序？

我想看看你的意思，但程序的输出是：

Contents of std::vector<MyClass> values
John Mike Dagobart John Mike Dagobart John Mike Dagobart John Mike 
Contents of HashTable hashtable
Mike 0
John 0
John 5
John 10
John 15
Mike 25
Dagobart 25
Dagobart 26
Mike 27
Dagobart 27

values size first 11
hash size fist 10
removing all bIsMarkedToDeleteMike Mike Mike Mike 
values size now 7
hash size now 7
Contents of value after removing elements 
John Dagobart John Dagobart John Dagobart John 
Contents of HashTable hashtable after delete Mike
Dagobart 25
John 0
Dagobart 26
John 15
John 10
John 5
Dagobart 27

0
0
Done

我不得不假设“第一次”是“ HashTable哈希表的内容”的部分。事实上，如果你仔细观察，似乎是“按桶分类”。很容易理解容器是逐桶迭代的。

事实上，在删除之后它不再与你的哈希/相等实现不匹配这一事实有很大关系（参见上文）。

  

2）我如何通过键自己走散列表并说出std :: cout每个散列到存储桶的项目，例如#if 0 #endif部分中的代码？

没有直接（公共API）方式。您可以使用hashtable.bucket（key）来构建用于调试目的的映射：

<强> Live On Coliru 

#include <vector>
#include <iostream>
#include <vector>
#include <map>
#include <functional>

#include <boost/intrusive/unordered_set.hpp>

namespace bic = boost::intrusive;

std::hash<std::string> hash_fn;

struct MyClass : bic::unordered_set_base_hook<bic::link_mode<bic::auto_unlink>>
{
    std::string name;
    int anInt1;
    mutable bool bIsMarkedToDelete;

    MyClass(std::string name, int i) : name(name), anInt1(i), bIsMarkedToDelete(false) {}

    bool operator==(MyClass const& o) const
    {
        return anInt1 == o.anInt1 && name == o.name;
    }

    struct hasher
    {
        size_t operator()(MyClass const& o) const
        {
            return hash_fn(o.name);
        }
    };
};

std::ostream& operator << (std::ostream& out, const MyClass& ac) {
    return out << ac.name << " " << ac.anInt1;
}

typedef bic::unordered_set<MyClass, bic::hash<MyClass::hasher>, bic::constant_time_size<false> > HashTable;

int main()
{
    std::vector<MyClass> values {
        MyClass { "Dagobart", 25 },
        MyClass { "Dagobart", 26 },
        MyClass { "Dagobart", 27 },
        MyClass { "John",     0  },
        MyClass { "John",     10 },
        MyClass { "John",     15 },
        MyClass { "John",     5  },
        MyClass { "Mike",     0  },
        MyClass { "Mike",     25 },
        MyClass { "Mike",     25 },
        MyClass { "Mike",     27 }
    };

    HashTable::bucket_type buckets[100];
    HashTable hashtable(values.begin(), values.end(), HashTable::bucket_traits(buckets, 100));

    std::cout << "\nDebugging buckets of hashtable\n";

    std::multimap<size_t, MyClass const*> debug_map;
    std::transform(hashtable.begin(), hashtable.end(), 
            std::inserter(debug_map, debug_map.end()), 
            [&](MyClass const& mc) { return std::make_pair(hashtable.bucket(mc), &mc); }
        );

    for (auto& entry : debug_map)
        std::cout << "Debug bucket: " << entry.first << " -> " << *entry.second << "\n";
}

打印

Debugging buckets of hashtable
Debug bucket: 16 -> Mike 27
Debug bucket: 16 -> Mike 25
Debug bucket: 16 -> Mike 0
Debug bucket: 21 -> Dagobart 27
Debug bucket: 21 -> Dagobart 26
Debug bucket: 21 -> Dagobart 25
Debug bucket: 59 -> John 5
Debug bucket: 59 -> John 15
Debug bucket: 59 -> John 10
Debug bucket: 59 -> John 0

当然输出取决于std::hash<std::string>的实际实现和散列表的调整。

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