typedef pair value_type;
value_type 被声明为 pair <const key_type, mapped_type> 但并不是简单的 pair <key_type, mapped_type>因为用一个非常量的迭代器或引用不能改变关联容器的Key。
| // map_value_type.cpp // compile with: /EHsc #include #include
int main( ) { using namespace std; typedef pair cInt2Int; map m1; map :: key_type key1; map :: mapped_type mapped1; map :: value_type value1; map :: iterator pIter;
// value_type can be used to pass the correct type // explicitly to avoid implicit type conversion m1.insert ( map :: 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. |