map::value_type 示例代码-优快云博客

本文链接：https://blog.youkuaiyun.com/FocusOnACE/article/details/6478999

本文展示了如何在 C++ 中使用 `map::value_type` 插入和访问映射元素。通过实例代码解释了 `value_type` 的作用，避免了隐式类型转换，并演示了插入和打印 map 中元素的关键步骤。

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// map_value_type.cpp
// compile with: /EHsc
#include <map>
#include <iostream>

int main( )
{
   using namespace std;
   typedef pair <const int, int> cInt2Int;
   map <int, int> m1;
   map <int, int> :: key_type key1;
   map <int, int> :: mapped_type mapped1;
   map <int, int> :: value_type value1;
   map <int, int> :: iterator pIter;
   // value_type can be used to pass the correct type
   // explicitly to avoid implicit type conversion
   m1.insert ( map <int, int> :: value_type ( 1, 10 ) );

   // Compare other ways to insert objects into a map
   m1.insert ( cInt2Int ( 2, 20 ) );
   m1[ 3 ] = 30;

   // Initializing key1 and mapped1
   key1 = ( m1.begin( ) -> first );
   mapped1 = ( m1.begin( ) -> second );

cout << "The key of first element in the map is "
<< key1 << "." << endl;

cout << "The data value of first element in the map is "
<< mapped1 << "." << endl;

   // The following line would cause an error because
   // the value_type is not assignable
   // value1 = cInt2Int ( 4, 40 );

   cout << "The keys of the mapped elements are:";
   for ( pIter = m1.begin( ) ; pIter != m1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout << "The values of the mapped elements are:";
   for ( pIter = m1.begin( ) ; pIter != m1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;
}

Output

The key of first element in the map is 1.
The data value of first element in the map is 10.
The keys of the mapped elements are: 1 2 3.
The values of the mapped elements are: 10 20 30.