hash_map中string為key的解決方法

當hash_map中使用string為key時，需使用者擴充套件名稱空間，否則報錯如下：

/usr/lib/gcc/x86_64-redhat-linux/3.4.5/../../../../include/c++/3.4.5/ext/hashtable.h:518: error: no match for call to `(const __gnu_cxx::hash<std::string>) (const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)'
表明編譯器錯誤，修改方法有如下幾個方案：

方案一：

自己寫一個標頭檔案，當使用string為key時，加入該標頭檔案即可。指明瞭不同編譯器下擴充套件形式。（必殺技）。檔案如下：

#ifndef HASH_MAP_HPP
#define HASH_MAP_HPP

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <string>

#if defined(_STLPORT_VERSION)
    #include <hash_map>
    #include <hash_set>
    using std::hash;
    using std::hash_map;
    using std::hash_set;
#else // not using STLPORT

    #ifdef __GNUC__
        #if __GNUC__ >= 3
            #include <ext/hash_map>
            #include <ext/hash_set>
            namespace __gnu_cxx {
                template <>
                struct hash<std::string> {
                    size_t operator()(const std::string& s) const {
                        unsigned long __h = 0;
                        for (unsigned i = 0;i < s.size();++i)
                            __h ^= (( __h << 5) + (__h >> 2) + s[i]);
                        return size_t(__h);
             
                    }
                   
                };
            }
            using __gnu_cxx::hash_map;
            using __gnu_cxx::hash;
        #else // GCC 2.x
            #include <hash_map>
            #include <hash_set>
            namespace std {
                struct hash<std::string> {
                    size_t operator()(const std::string& s) const {
                        unsigned long __h = 0;
                        for (unsigned i = 0;i < s.size();++i)
                            __h ^= (( __h << 5) + (__h >> 2) + s[i]);
                        return size_t(__h);
                    }
                };
            };
            using std::hash_map;
            using std::hash_set;
            using std::hash;
        #endif // end GCC >= 3
    #elif defined(_MSC_VER) && ((_MSC_VER >= 1300) || defined(__INTEL_COMPILER))
        // we only support MSVC7+ and Intel C++ 8.0
        #include <hash_map>
        #include <hash_set>
        namespace stdext {
            inline size_t hash_value(const std::string& s) {
                unsigned long __h = 0;
                for (unsigned i = 0;i < s.size();++i)
                    __h ^= (( __h << 5) + (__h >> 2) + s[i]);
                return size_t(__h);
            }
        }
        using std::hash_map; // _MSC_EXTENSIONS, though DEPRECATED
        using std::hash_set;
    #else
        #error unknown compiler
    #endif //GCC or MSVC7+
#endif // end STLPORT
#endif /* ifndef HASH_MAP_HPP */

方案二：

想簡單一些，快速使用，可遵照如下寫法：

#include <iostream>
#include <string>
#include <ext/hash_map>
using namespace std;
using namespace __gnu_cxx;

namespace __gnu_cxx
{
    template<> struct hash<const string>
    {
        size_t operator()(const string& s) const
        { return hash<const char*>()( s.c_str() ); } //
__stl_hash_string
    };
    template<> struct hash<string>
    {
        size_t operator()(const string& s) const
        { return hash<const char*>()( s.c_str() ); }
    };
}

int main( void )
{
    hash_map<string,int> test;
    test["abc"] = 1;
    cout << test["abc"] << endl;
    system( "pause" );
}

方案三：
再擴充套件一些，當使用long做key時，同樣擴充套件：

#ifndef EXT_HASH_FUNCTION_HPP_INCLUDED
#define EXT_HASH_FUNCTION_HPP_INCLUDED

#ifdef __GNUC__
namespace __gnu_cxx
{
        template<>
        struct hash<long long>
        {
                size_t operator()(long long __x) const
                {
                        if (sizeof(__x) == sizeof(size_t))
                                return __x;
                        else
                                return (__x >> 32) ^ (__x & 0xFFFFFFFF);
                }
        };

        template<>
        struct hash<unsigned long long>
        {
                size_t operator()(unsigned long long __x) const
                {
                        if (sizeof(__x) == sizeof(size_t))
                                return __x;
                        else
                                return (__x >> 32) ^ (__x & 0xFFFFFFFF);
                }
        };

        template<typename Traits, typename Allocator>
        struct hash<std::basic_string<char, Traits, Allocator> >
        {
                size_t operator()(const std::basic_string<char, Traits, Allocator>& __s) const
                {
                        return __stl_hash_string(__s.c_str());
                }
        };

}
#endif

#endif//EXT_HASH_FUNCTION_HPP_INCLUDED