hash_map::operator[]

在 hash_map 中插入具有指定键值的元素。

Type& operator[](
   const Key& _Key
);
Type& operator[](
   Key&& _Key
);

参数

返回值

为 Insert 的元素数据值的引用。

备注

如果关键参数未找到值，则它与数据类型的默认值。插入。

operator[] 可用于将元素插入到 hash_map m。

m[_Key] = DataValue;

如果 DataValue 是元素的 mapped_type 值用键值的 _Key。

当使用 operator[] 插入元素，则返回的引用不指示插入是否更改预先存在的元素或创建新查询。 成员函数 查找插入 和来确定用指定的键的元素是否在插入之前已存在。

示例

// hash_map_op_ref.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>
#include <string>

int main( )
{
   using namespace std;
   using namespace stdext;
   typedef pair <const int, int> cInt2Int;
   hash_map <int, int> hm1;
   hash_map <int, int> :: iterator pIter;
   
   // Insert a data value of 10 with a key of 1
   // into a hash_map using the operator[] member function
   hm1[ 1 ] = 10;

   // Compare other ways to insert objects into a hash_map
   hm1.insert ( hash_map <int, int> :: value_type ( 2, 20 ) );
   hm1.insert ( cInt2Int ( 3, 30 ) );

   cout  << "The keys of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;

   // If the key already exists, operator[]
   // changes the value of the datum in the element
   hm1[ 2 ] = 40;

   // operator[] will also insert the value of the data
   // type's default constructor if the value is unspecified
   hm1[5];

   cout  << "The keys of the mapped elements are now:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are now:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;

   // opperator[] will also insert by moving a key
   hash_map <string, int> hm2;
   string str("a");
   hm2[move(str)] = 1;
   cout << "The moved key is " << hm2.begin()->first
      << ", with value " << hm2.begin()->second << endl;
}

Output

The keys of the mapped elements are: 1 2 3.
The values of the mapped elements are: 10 20 30.
The keys of the mapped elements are now: 1 2 3 5.
The values of the mapped elements are now: 10 40 30 0.
The moved key is a, with value 1.

要求

标头: <hash_map>

**命名空间：**stdext

请参见

参考

hash_map 类

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