c++ hash_map用法总结

c++ STL库里有自定义的hash_map 方法，但是使用起来并不是那么方便

hash_map主要的方法有

find(),insert()

我结合官方API说明一下他们的用法

一、需要特别注意的地方，

1.头文件的引用

2.如何插入一个<key,value>键值对（参考一下代码）

3.find（）的返回值

4.如何获取某一个key值相应的value值

 hm1_RcIter -> second

分别用。first，和，second，指代key和value的值

// hash_map_find.cpp// compile with: /EHsc#define _DEFINE_DEPRECATED_HASH_CLASSES 0#include <hash_map>#include <iostream>int main( ){   using namespace std;   using namespace stdext;   hash_map <int, int> hm1;   hash_map <int, int> :: const_iterator hm1_AcIter, hm1_RcIter;   typedef pair <int, int> Int_Pair;   hm1.insert ( Int_Pair ( 1, 10 ) );   hm1.insert ( Int_Pair ( 2, 20 ) );   hm1.insert ( Int_Pair ( 3, 30 ) );   hm1_RcIter = hm1.find( 2 );   cout << "The element of hash_map hm1 with a key of 2 is: "        << hm1_RcIter -> second << "." << endl;   // If no match is found for the key, end( ) is returned   hm1_RcIter = hm1.find( 4 );   if ( hm1_RcIter == hm1.end( ) )      cout << "The hash_map hm1 doesn't have an element "           << "with a key of 4." << endl;   else      cout << "The element of hash_map hm1 with a key of 4 is: "           << hm1_RcIter -> second << "." << endl;   // The element at a specific location in the hash_map can be found    // using a dereferenced iterator addressing the location   hm1_AcIter = hm1.end( );   hm1_AcIter--;   hm1_RcIter = hm1.find( hm1_AcIter -> first );   cout << "The element of hm1 with a key matching "        << "that of the last element is: "        << hm1_RcIter -> second << "." << endl;}

Output

The element of hash_map hm1 with a key of 2 is: 20.The hash_map hm1 doesn't have an element with a key of 4.The element of hm1 with a key matching that of the last element is: 30.

二、hash_map使用string和long long做key的问题

当hash_map中使用string为key时，需用户扩展命名空间，否则报错如下：

/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> >&)'
表明编译器错误

解决方法：

#include <ext/hash_map>
namespace __gnu_cxx
{
    template<> struct hash< std::string >
    {
        size_t operator()( const std::string& x ) const
        {
            return hash< const char* >()( x.c_str() );
        }
    };

    template<> struct hash<long long>
    {
        size_t operator()(long long x) const
        {
            return x;
        }
    };
}