c++如何理解map对象的value_type是pair类型

map 是以 pair形式插入的。

map中的元素的类型value_type
typedef pair<const Key, Type> 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。

#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.