c++ hash_map用法总结

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本文总结了C++ STL库中的hash_map使用方法，包括find()和insert()的使用，强调了头文件引用、键值对插入、查找返回值及获取value的注意事项。还提到了使用string作为key时可能出现的编译错误及其解决方法。

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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;
}
};
}