简单C ++ hash_set示例 [英] simple C++ hash_set example

问题描述

我是C ++和STL的新手。我遇到以下简单的哈希集存储自定义数据结构的例子：

I am new to C++ and STL. I am stuck with the following simple example of a hash set storing custom data structures:

#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，我得到以下错误消息：

when I try to compile it, I get the following error message:

/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

我在做什么错了？

推荐答案

这么近！在输出中的最后一个错误显示你的 hash_trip 程序应宣布常量：

So close! The last error in your output reveals your hash_trip routine should be declared const:

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

你可能需要对做同样的事情eq_trip 。另外，我建议通过不断的引用传递参数给这些功能，以避免您在传递数据的不必要的复制：

You'll probably need to do the same thing for eq_trip. Also, I would recommend passing the arguments to these functions by constant reference to avoid an unnecessary copy of the data you're passing:

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

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