c++ - 使用 hash_map,string> 失败，为什么？

Question

我已经实现了这段代码，但是编译失败(VC2008 Express Ed.):
现在所有代码都在这里。

#include <stdio.h>
#include <vector>
#include <string>
#include <hash_map>

using namespace std;
using namespace stdext;

typedef vector type_key;

int main(int argc, char* argv[])
{
    type_key key;

    hash_map<type_key, string> map_segment;
    hash_map<type_key, string>::iterator iter_map_segment;

    iter_map_segment = map_segment.find(key);

    return 0;
}

使用unordered_map也会出现同样的错误。

但不会发生用 map 错误替换容器的情况。
可以做些什么来纠正？
谢谢。

[编辑]
错误信息:

------ Build started: Project: map_key_test, Configuration: Debug Win32 ------
Compiling...
map_key_test.cpp
c:\program files\microsoft visual studio 9.0\vc\include\xhash(75) : error C2440: 'type cast' : cannot convert from 'const type_key' to 'size_t'
        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
        c:\program files\microsoft visual studio 9.0\vc\include\xhash(128) : see reference to function template instantiation 'size_t stdext::hash_value<_Kty>(const _Kty &)' being compiled
        with
        [
            _Kty=type_key
        ]
        c:\program files\microsoft visual studio 9.0\vc\include\xhash(127) : while compiling class template member function 'size_t stdext::hash_compare<_Kty,_Pr>::operator ()(const _Kty &) const'
        with
        [
            _Kty=type_key,
            _Pr=std::less<type_key>
        ]
        c:\program files\microsoft visual studio 9.0\vc\include\hash_map(78) : see reference to class template instantiation 'stdext::hash_compare<_Kty,_Pr>' being compiled
        with
        [
            _Kty=type_key,
            _Pr=std::less<type_key>
        ]
        c:\program files\microsoft visual studio 9.0\vc\include\xhash(191) : see reference to class template instantiation 'stdext::_Hmap_traits<_Kty,_Ty,_Tr,_Alloc,_Mfl>' being compiled
        with
        [
            _Kty=type_key,
            _Ty=std::string,
            _Tr=stdext::hash_compare<type_key,std::less<type_key>>,
            _Alloc=std::allocator<std::pair<const type_key,std::string>>,
            _Mfl=false
        ]
        c:\program files\microsoft visual studio 9.0\vc\include\hash_map(88) : see reference to class template instantiation 'stdext::_Hash<_Traits>' being compiled
        with
        [
            _Traits=stdext::_Hmap_traits<type_key,std::string,stdext::hash_compare<type_key,std::less<type_key>>,std::allocator<std::pair<const type_key,std::string>>,false>
        ]
        c:\users\salamon\documents\visual studio 2008\projects\map_key_test\map_key_test.cpp(17) : see reference to class template instantiation 'stdext::hash_map<_Kty,_Ty>' being compiled
        with
        [
            _Kty=type_key,
            _Ty=std::string
        ]
Build log was saved at "file://c:\Users\Salamon\Documents\Visual Studio 2008\Projects\map_key_test\Debug\BuildLog.htm"
map_key_test - 1 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[编辑]
完整的解决方案:

#include <stdio.h>
#include <vector>
#include <string>
#include <hash_map>
using namespace std;
using namespace stdext;

class Vector : public vector<int>
{
public:
    operator size_t() const { return (*this).size(); };
};
typedef Vector type_key;

struct less_vector
{
    bool operator()(const Vector & x, const Vector & y) const
    {
        if ( x != y )
            return true;

        return false;
    }
};

struct greater_vector
{
    bool operator()(const Vector & x, const Vector & y) const
    {
        if ( x == y )
            return true;

        return false;
    }
};

int main(int argc, char* argv[])
{
    Vector key;
    string str;

    hash_map<Vector, string, hash_compare <Vector, less_vector > > map_segment;
    hash_map<Vector, string, hash_compare <Vector, greater_vector > >::iterator iter_map_segment;

    //
    key.push_back(0);
    key.push_back(1);
    key.push_back(2);
    str = "012";
    map_segment [key] = str;
    //
    key.clear();
    key.push_back(1);
    key.push_back(0);
    key.push_back(2);
    str = "102";
    map_segment [key] = str;
    //
    key.clear();
    key.push_back(2);
    key.push_back(1);
    key.push_back(0);
    str = "210";
    map_segment [key] = str;
    //
    key.clear();
    key.push_back(1);
    key.push_back(0);
    key.push_back(2);

    iter_map_segment = map_segment.find(key);

    return 0;
}

Answer 1

首先，确保您包含的 header 确实存在于您的安装中。

其次，hash_map在stdext中命名空间，请确保通过将其明确指定为 stdext::hash_map 来使该名称可见，或通过 using指令。

第三，代码中 hash_map 的模板参数在哪里？我只看到hash_map而不是 hash_map<vector<int>,string> .

第四，我不确定 std::vector 是否可以像这样用作散列映射的散列键。您需要为您的 key 定义一个哈希函数(通过实现一个 hash_compare 函数对象并将其作为第三个模板参数传递给 hash_map)。无论如何，按值存储这些 vector 并不是一个好主意，相反，您可能应该考虑使用指向这些 vector 的指针作为键。