简单的 C++哈希集示例

Question

我是 C++ 和 STL 的新手。我遇到了以下存储自定义数据结构的哈希集的简单示例：

#include <iostream>
#include <ext/hash_set>

using namespace std;
using namespace __gnu_cxx;

struct trip {
    int trip_id;
    int delta_n;
    int delta_secs;

    trip(int trip_id, int delta_n, int delta_secs){
        this->trip_id = trip_id;
        this->delta_n = delta_n;
        this->delta_secs = delta_secs;
    }
};


struct hash_trip
{
    size_t operator()(const trip t)
    {
        hash<int> H;
        return H(t.trip_id);
    }
};

struct eq_trip
{
    bool operator()(const trip t1, const trip t2) {
        return (t1.trip_id==t2.trip_id) &&
        (t1.delta_n==t2.delta_n) &&
        (t1.delta_secs==t2.delta_secs);
    }
};

int main()
{
    hash_set<trip, hash_trip, eq_trip> trips;

    trip t  = trip(3,2,-1);
    trip t1  = trip(3,2,0);

    trips.insert(t);

}

当我尝试编译它时，我收到以下错误消息：

/usr/include/c++/4.2.1/ext/hashtable.h: In member function ‘size_t __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::_M_bkt_num_key(const _Key&, size_t) const [with _Val = trip, _Key = trip, _HashFcn = hash_trip, _ExtractKey = std::_Identity<trip>, _EqualKey = eq_trip, _Alloc = std::allocator<trip>]’:
/usr/include/c++/4.2.1/ext/hashtable.h:599:   instantiated from ‘size_t __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::_M_bkt_num(const _Val&, size_t) const [with _Val = trip, _Key = trip, _HashFcn = hash_trip, _ExtractKey = std::_Identity<trip>, _EqualKey = eq_trip, _Alloc = std::allocator<trip>]’
/usr/include/c++/4.2.1/ext/hashtable.h:1006:   instantiated from ‘void __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::resize(size_t) [with _Val = trip, _Key = trip, _HashFcn = hash_trip, _ExtractKey = std::_Identity<trip>, _EqualKey = eq_trip, _Alloc = std::allocator<trip>]’
/usr/include/c++/4.2.1/ext/hashtable.h:437:   instantiated from ‘std::pair<__gnu_cxx::_Hashtable_iterator<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>, bool> __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::insert_unique(const _Val&) [with _Val = trip, _Key = trip, _HashFcn = hash_trip, _ExtractKey = std::_Identity<trip>, _EqualKey = eq_trip, _Alloc = std::allocator<trip>]’
/usr/include/c++/4.2.1/ext/hash_set:197:   instantiated from ‘std::pair<typename __gnu_cxx::hashtable<_Value, _Value, _HashFcn, std::_Identity<_Value>, _EqualKey, _Alloc>::const_iterator, bool> __gnu_cxx::hash_set<_Value, _HashFcn, _EqualKey, _Alloc>::insert(const typename __gnu_cxx::hashtable<_Value, _Value, _HashFcn, std::_Identity<_Value>, _EqualKey, _Alloc>::value_type&) [with _Value = trip, _HashFcn = hash_trip, _EqualKey = eq_trip, _Alloc = std::allocator<trip>]’
try.cpp:45:   instantiated from here
/usr/include/c++/4.2.1/ext/hashtable.h:595: error: passing ‘const hash_trip’ as ‘this’ argument of ‘size_t hash_trip::operator()(trip)’ discards qualifiers

我做错了什么？

Answer 1

您应该研究使用 STL 的 TR1 扩展，即

• unordered_map
• unordered_set
• unordered_multimap
• unordered_mutliset

大多数现代 C++ 编译器都附带这些扩展，因此不需要使用非标准类，例如 hash_set 等。

• http://en.wikipedia.org/wiki/Unordered_map_%28C%2B%2B_class% 29
• http://publib.boulder.ibm.com/infocenter/comphelp/v9v111/index.jsp?topic=/com.ibm.xlcpp9.aix.doc/standlib/stl_unordered_map.htm
• http://www.cplusplus.com/reference/unordered_set/unordered_set/

Answer 2

这么近！输出中的最后一个错误表明您的 hash_trip 例程应声明为 const：

size_t operator()(const trip t) const // note the ending 'const'
{
    //...
}

您可能需要对 eq_trip 执行相同的操作。另外，我建议通过常量引用将参数传递给这些函数，以避免对所传递的数据进行不必要的复制：

size_t operator()(const trip& t) const // note the '&'
{
    //...
}